A March afternoon, on location in Manhattan. It is the day after the Oscars, and, relaxing in his trailer, Edward Norton explains why he wasn't partying, or even watching, the night before.

"If you go to the Oscars once or twice, you quickly get rid of the notion that it's anything romantic. It can be fun," he says, "if you have your family and friends come along, and you get a little loaded and have a laugh at the whole thing. That's the only thing to do - to treat it as an experience of complete facades and nonsense. You can treat it like what it is: ether."

He's been to the ceremony four times: twice when he was nominated for best actor, once as a presenter, and then with his ex, Salma Hayek, for Frida, which he co-wrote.

"This morning I overheard the girls in the make-up trailer. They watched last night and couldn't remember who won best supporting actress. The point being that it's not in itself contemptible, it's just much more irrelevant than what gets whipped up around it would lead you to believe. It's become this orgy of self-congratulation that runs from late November to March and begins with a series of critics awards which have proliferated like dandelions. I think, on a fundamental level, it's inappropriate for people to get so involved with the critics who write about them.

"I remember the first time I was up for one of those broadcast-critics awards. I caught eyes with Daniel Day-Lewis and he gave me this look of 'Can you believe this?' Now the Screen Actors Guild awards are televised, the Golden Globes are televised, and people go because they feel they have to, and by the time they get to the Oscars, the reason you see everybody with such tight faces is because they're frankly embarrassed to be sitting there - again.

"That's the part that becomes contemptible. It's excessive. With all that's going on in the world, continuing to congratulate yourselves and each other only reinforces the idea that these are self-aggrandising, vain, indulged people."

Norton rarely gives interviews. It's not that he hides from public view, he just doesn't have the time for it. While most actors are complaining about how hard they work - usually three times a year for six weeks at a stretch - Norton is industrious on an industrial scale, and movies aren't enough to occupy his intellect. Even the subject of an awards season elicits a comprehensive response. "I think it would be good if the artists could get together and just put the brakes on it all and say, 'This is too much. It's chewing up our lives and our time. It's making us look like morons and we need to chill this out.'

"You cannot imagine the effort and money that goes into acquiring these awards. Millions of dollars are getting spent. I think it would rescue some of the integrity if they'd put a clamp on the campaigning."

That's not to say he doesn't think awards are earned. "Phil Hoffman - he deserves it once a year. For me, I'd be happy if they gave it to Denzel Washington every year. He's that good."

Time has passed. It's dark outside. He has been at work since 7.30am, but he is not done yet. Throughout the day I have asked: Aren't you cold? "No." Tired? "No." Another shot is being set up for this movie - Pride & Glory - and the conversation turns to a personal area.

In 2000 Norton starred in a romantic comedy, which he also directed and produced, called Keeping the Faith, about a priest and a rabbi who fall in love with the same girl. It is dedicated to his mother, Robin, who died of brain cancer in 1997. He leans back on the sofa and explains his discomfort with this personal line of questioning, which in turn explains the rarity of his giving interviews. "I've learnt the hard way that there's nothing I like about having my most personal experiences become anecdotal. There's absolutely no spiritual upside for me in having a lot of strangers know about my life.

"Have you read that Hemingway story - Soldier's Home?" he asks. "It's about a soldier back from the first world war realising that sitting in places and telling people about the experiences he's had over there is giving him this incredibly sick feeling, because he feels he's sold out. That has kind of been my experience every time."

In Hemingway's story, the character, Krebs, felt compelled to exaggerate his stories because he thought it was what people wanted to hear, and this desire to satisfy, to need to be listened to, resulted in the sickening feeling.

But in Norton's case, the feeling of having sold out has less to do with personal shame than the professional consequence: the less we know of him personally, the more we believe in the role. "Everybody goes through certain experiences and, if you're lucky, even with the worst things, you come out of them and they induce in you an altered perspective on the relative importance of things. So when you talk about things like the Oscars, it's very hard to take it too seriously." With tragedy comes perspective. "It dials the volume down on everything that stressed you out previously.

"We're very disconnected from fundamental things," he says. "Only wealthy cultures have the luxury of worrying about face creams that prevent ageing. Beauty, fashion - they're the indulgences of the wealthiest cultures, and I think that along with that comes a tucking under of things you don't want to confront. The more people sell you the idea of spiritual peace through what you drive and how you look and how you live, the less connected you become."

This theme is what attracted him to Fight Club. It was a seminal role and even if he hadn't starred in it, he says, he would be a fan. "It still makes me laugh - that part where, if I could just get that last unit from Ikea in place, I know that I'll be calm. It cracks me up. Fight Club was so much about the hilarious chagrin of recognising what a slave you are to consumer advertising - there's no way you could not relate.

"I cannot tell you the number of things I get sent - PhD theses, psychology papers, divinity school papers. It's [Fight Club] become this touchstone where people can project almost anything they want."

Norton sees his latest film, Down in the Valley, as a companion piece to Fight Club. It is, in part, a parable set in the San Fernando Valley in Los Angeles. "I think it's about the same things. How a certain part of modernity has bent people and left them extremely adrift and disconnected from the adult world they're expected to engage in.

"It asks you to confront your feelings about what's transpired, and confront mixed emotions. That is life - it is in equal measure beautiful and poetic, but it's also painful."

Edward Norton reveals himself by what he doesn't do. He rarely walks the red carpet, doesn't network the studios, or smile toothily on camera to Oprah. You won't find him in the gossip magazines, at the autumn collections, or travelling with an entourage of minders, dieticians and personal trainers. He doesn't have a PR because he doesn't desire the spotlight - unless, and unusually, like now, the interest expressed in him goes beyond the celebrity.

I'd been chasing an interview for a while and when his agent told me he would call me himself, he did. He wanted to feel assured that the interview would be a worthwhile interruption to his work - not an invasion of his life.

"It's gotten to the point where now, when I read about myself, if I'm only mildly nauseous I consider that an upside," he joked. But he understood that the usual star turn - a sit-down for an hour over lunch - would not produce any meaningful result and he wanted to ensure there was time. He'd check his schedule and get back to me. E-mails were exchanged, and a week or so later, he called again.

"Do you have any time this afternoon?" he asked. More time was offered for later, perhaps on the set. But for now, he's free for a couple of hours. Twenty minutes later we meet on a corner in the West Village, where he lives alone, and we walk around looking for somewhere discreet to hang out. He is tall and slim and wearing an overcoat. When we find somewhere, only one other table is occupied - by the actress Ellen Barkin. They greet each other warmly before we take a booth and start to talk.

It's a much-abused cliché that Norton is ranked "the finest actor of his generation". "Intelligent", too, is one of those words that's been so overused that it's hard to know what it means any more. The perception that he is serious, sombre and intimidating is largely due to his iconic performances in Primal Fear, Fight Club and American History X. In these films, he tapped so effectively into rage that it seems impossible that he would not inhabit this quality in real life. But his placid manner is one of his most salient features. He is soft-spoken, even-tempered; you can see him working out a thought, processing it like a philosopher. Even if he's talking about something he's talked about a million times before, you can tell he's trying to find a different way to say it.

Though he has a comedic and goofy side - as seen in Woody Allen's Everyone Says I Love You - for the most part he is associated with tragic, conflicted characters.

The core of American History X is the sorrow of seeing someone destroy themselves through rage - seeing all that he could have been, but it's too late. So where does the connection to that character come from?

"I don't personally have a lot of rage," he says. "I can't deconstruct it. I've always been able to channel it very effectively." He smiles. "My brother and sister think it's fascinating and slightly comical and mystifying that people relate to me as an intimidating presence in these films."

The conversation traverses a multitude of subjects. Norton is interested in history, ideas and character. He has knowledge, depth and opinions.

On Theodore Roosevelt: "He had a voracious mind and he was a nuclear furnace of energy and productivity. He would take a vacation - a rare vacation - and on it he writes the definitive biography of Thomas Hart Benton. I think he had a keen sense of social justice and an encompassing vision of what America's potentials were. I think he was a blowhard and maybe had a little bit too much blood lust for war, but he definitely had an egalitarian sense of America. He was a bona-fide reformer and very quick to champion the oppressed against the entrenched corporate interests."

Or on Hollywood's idea of "historical" film-making: "Amistad bothers me a lot more than Troy. In a world where no significant film has been made about the slave trade, when you choose to make a film about that part of history and you choose as the focus the Amistad incident, which is this completely anomalous incident of strange justice, I think the burden is very heavy on you, to make sure you are not suggesting that that strange piece of justice on any level redeems that history. I felt that they failed terribly in that regard.

"I remember Spike Lee saying that if he had done to Schindler's List what Spielberg did to the Amistad incident, someone would have hung him from a light pole, and I agree with him. I think in a three-hour film there was 15 minutes depicting the horror of the slave trade and it was the best 15 minutes of the film."

The conversation moves to books. He says Mountains beyond Mountains: The Quest of Dr Paul Farmer, a Man Who Would Cure the World, by Tracy Kidder, was one of the most significant reading experiences of his life. "It had paradigm-shifting ideas about poverty and healthcare. Here was someone with no ambition for fame or money. His ambition is to fundamentally change the way people look at the most intrinsic problem - poverty and health. I came away from the book feeling it had vaulted him to the ranks of the Gandhis and Martin Luther Kings. You read that and you go, 'What the hell am I doing with my life?'"

This is a question we will return to. What Edward Norton is doing with his life is evidenced in his admiration of Roosevelt, champions of social justice, and the use of power to challenge and change the status quo.

When asked if he could handle being obscure, he considers for a while before answering. "Well, the knee-jerk reaction is to say, 'Yeah, definitely,' because I like privacy and I also like being able to observe. The loss of anonymity makes it harder to do that. But on the other hand, there are all sorts of things you can do with being well known. There are balances. I wouldn't trade it because fame - recognition - can be leveraged into extremely positive things."

He is not a worrier, but if he worries about anything it's that he is working too much and may not have the discipline to take a break to do things that he's always wanted to do. He is never idle. He'll put scuba gear in his plane and fly to Mexico. He flies a Cessna, and when asked if he owns it, he looks embarrassed by the extravagance and tells me it costs less than a Ferrari.

A self-described 'pack rat', a hoarder, Norton has saved his answering-machine message tapes for years, voices from friends who have died, or just funny, important moments.

"I archive more than I should," he says. He keeps written journals too. When he rereads them, what does he learn about himself? "Sometimes I think you tell yourself you've learnt certain things - you know, those moments when you really see the gulf between the vision of yourself you project and the actuality. It's pretty fascinating how much of our behaviour is based on compulsion rather than conscious choice. I think we can learn how to rewire our behaviour - just not as easily as we think. It takes twice as many passes through an experience as you think it will."

His grandfather was a hard act to follow. James Rouse, confidante of presidents, recipient of America's highest accolades, was a maverick urban planner who believed cities should enhance their residents' lifestyle. He transformed America's landscape, and championed the rejuvenation of abandoned docks and urban slums. Blighted waterfronts in Boston and Manhattan became Faneuil Hall and the South Street Seaport - hotspots for dining, shopping and cultural activity. Cities worldwide embraced his vision. Because of him, London has Covent Garden. Then he turned his attention to the poor. Believing that affordable, attractive housing would encourage them out of poverty, he founded the Enterprise Foundation (now Enterprise Community Partners), raised $6 billion, and has built nearly 100,000 homes across America. He even built a town, Columbia, in Maryland, designed to his own principles. In 1995, a year before he died, at 81, he was awarded the Presidential Medal of Freedom by Bill Clinton.

You need to know Rouse's place in contemporary American history if you want to begin to understand what makes Edward Norton tick. His father, Edward Senior, was an environmental lawyer long before the environment became a global issue and is now the deputy director of the Asia-Pacific region of the Nature Conservancy, an international non-profit organisation.

Born in Boston in 1969, Norton grew up in Columbia, Maryland - the town built by his grandfather - graduated from Yale in 1991 with a history degree, and moved to New York. He learnt to speak Japanese and worked for Enterprise in Manhattan and in Osaka, Japan. But he had enjoyed acting at school and college and was drawn back to the theatre. He was a waiter for a while, yet it wasn't long before he was cast in Edward Albee's play Fragments. Soon after came his breakthrough role in Primal Fear.

Norton's acting achievements to date might be enough to satisfy the most ambitious young man. But his acting career is only scratching the surface. The other Edward Norton is rooted in his genes, dedicated to the family business of campaigning. In the smug, often shallow world of celebrity, Norton is a rarity - someone who doesn't just talk about changing the world, but actually commits to it.

A quick CV reads like this: he's a trustee on the board of Enterprise; his production company makes documentaries about environmental campaigns; he's established the Peacemakers Fund at Yale in response to 9/11, which gives grants to students to travel in the Middle East; he's a board member of a group that is turning almost two miles of abandoned railway line through Manhattan's industrial West Side into a public park. He also pours time and money into globally diverse causes, from the Yunnan Great Rivers conservation project in China to the Maasai Wilderness Conservation Trust to the Wilderness Society and the Southern Center for Human Rights - a public-interest law firm that enforces people's rights in the criminal justice system in the southern states. And it's not just his patronage, a name on a piece of paper - he's an activist following in his family's footsteps.

Solar power is another passion. "It was one of those moments where you go, 'This was so easy. It was so easy. Why isn't everybody doing this?'"

We are back in his trailer, between scenes, and he is talking about the solar technology he installed in his LA home, and the idea it gave him. "I started thinking. I'm involved in the affordable housing project and I know that one of the issues is keeping costs down - the people who need this technology the most are the people who can afford their utility bills the least. What if we could create a data pool of 50-100 low-income homeowners who have solar and we track it and see the impact on low-income families?"

He saw the opportunity to collect data that could be used to lobby legislators. "I went to BP Solar and said, 'Here's what I want...'" He devised an exchange. For every solar panel made by BP purchased by someone high profile, BP would donate matching panels to low-income homes.

"The first year we got about 25 people - some were friends, like Brad Pitt and Danny DeVito, and some were out of the blue like Daryl Hannah and Alicia Silverstone. We're accumulating a bunch of low-income homeowners with solar systems, and Enterprise is tracking the impact on their bills and what they're doing with that money.

"Over the next 10 years, billions of dollars are going to get fed out in different forms - loan funds to affordable housing developers, direct solar subsidies... And we are up there, Enterprise and Global Green, another group I work with, lobbying lawmakers to say, 'A dollar of solar subsidy aimed at a low-income family is a taxpayer's dollar better spent.'

"By helping low-income families spend less on utilities, they're able to come further off social welfare and move further away from the margins, and there's lots of cost benefits to the public - not to mention the environment. The problem is, there's not that much information to take it away from theory and prove it. That's why we're trying to create this data pool. We're arguing that there should be a significant amount of funds set aside for affordable housing."

It sounds like it must soak up a lot of time and energy. But he shrugs off having to choose between acting and activism. He doesn't see them as mutually exclusive.

"It was a lot of work to set it up, and lobbying, but then it downshifts into what I do now, which is keep track of it.

"You see how it is here. We break for an hour, you can get a lot done in an hour. The truth is, making movies, there's a lot of down time. It's a very fragmented process. You can't walk around 16 hours a day in character."

Moments later we're outside, on 176th street in Washington Heights. Location trucks, thick black cables, and bundled-up crew members with walkie-talkies are clustered around a narrow alleyway where Norton is shooting a scene with a hyperactive five-year-old who isn't paying attention. "C'mon, pal," he says, gently. "Let's get through this scene."

In the film Norton plays a homicide detective and, unlike his co-star Colin Farrell, whose presence signals "movie star", Norton quietly blends in. He seems strikingly normal.

So how does someone with such a committed social conscience, who not only avoids being the centre of attention but vigorously rejects it, reconcile his involvement in a business permeated by insecurity and inflated egos? He may remove himself from the excess but he still encounters it. Hollywood can be toxic. And when that happens, how is it managed? Is he amused by it? Disgusted? Immune? "It's your own choice what you choose to engage in," he says. "The working part of making movies, I enjoy it - even when it's frustrating. It's collaborative. Everyone has their own ideas, instincts and egos, and sometimes you'll run into some really silly egotistical behaviour - people who say, 'I need my M&M's cold or I won't come out of my trailer,' but it's rare.

"A puffed-up attitude really stands out. You can talk to crews who have a story about someone, but the reason it's a story is because it's an anomaly. When people run into it, they're flabbergasted. In general, the people I consider high-quality artists cultivate environments that don't really tolerate that kind of behaviour. No one's gonna pull that stuff on Milos Forman, you know?"

There is a clarity of purpose about Norton that explains him. It is a dynamic work ethic combined with an enquiring mind. He is not one to opt for pat answers or redemptive endings just because it's easier. Even when he leaves, it's open to interpretation. Has our time ended? Or if I stay in this trailer, will it continue? Edward Norton will keep going until the job is done.