If you watch Ryan Gosling's recent films back to back, you begin to wonder how he gets away with it: How, in this celebrity-fuelled age, an Oscar-nominated 26-year-old who has been living and working in Los Angeles for the past 10 years has managed to become a varied, complicated, powerful actor. How on earth has he evaded the forces that attempt to mould people like him into the next Brad Pitt?

'It's not like people looked at me and thought: Here's a movie star,' Gosling says when I meet him in New York. 'Most of my life I've tried to prove people wrong about me, and now [after Half Nelson, a film in which he plays a crack-addicted school teacher, which has earned him an Oscar nomination for best actor] I kind of have to prove some people right.' This is troubling to Gosling, whose anti-establishment instincts don't know what to do with the new signals. 'People are being very supportive, and that's against my plan,' he says, shaking his head, 'I was planning on being ostracised!'

We are sitting at a corner table in the Cupping Room Cafe in SoHo, where the air is thick with the smell of freshly ground coffee. Gosling has a mug locked in a firm embrace, and asks for a steady stream of refills. In the past six years he has played a Jewish neo-Nazi in The Believer (2001), a homicidal flirt in Murder By Numbers (2002), an imprisoned teenager in The United States of Leland (2003), a handsome lead in the wartime romance The Notebook (2004), and now, in Half Nelson, a putative role model who has lost control of his life. Gosling is extraordinary - in the first and last of those performances particularly - and virtually unrecognisable from one role to the next.

The Believer, which was so controversial that despite winning the Grand Jury Award at Sundance it never had a US cinema release - gave Gosling the role of a lifetime. Now that it has been released on DVD it may be more helpful in cementing Gosling's Half Nelson-inspired reputation than it was at its delicate outset. Based on the true story of a Ku Klux Klan member who was revealed by a New York Times reporter to be Jewish, The Believer offers up a central character so beautifully written, Gosling says, 'that you could put any actor in it'. That's not quite true: as Danny Balint, Gosling assumes all the articulacy, self-hatred, violence and glee of a truly fearsome young man, and gives Balint more dimensions than you'd care to contemplate. When he head-butts someone in slow motion, we see not just fury, but an unmistakable glint of ecstasy in his eyes.

Gosling got the part by accident. He was helping a friend with his lines, and was so impressed with the script that he begged for an audition himself. Nothing he had done in the past would have suggested he was capable of such a performance. His role as Young Hercules in the TV series, and as one of the presenters of The Mickey Mouse Club were still very recent memories. But Believer director Henry Bean eventually agreed to see him in his lunch hour. He thought Gosling's own upbringing contained the kinds of complexities that would see him through it. Gosling explains: 'Henry loves the dark horse.'

Gosling grew up in a small paper mill town in Canada; his family are Mormons, and his father and uncle worked at the mill. 'We were brought up pretty religious,' he recalls. 'My mother admits it: She says, you were raised by a religious zealot. She's different now, but at the time, it was a part of everything - what they ate, how they thought ... ' At school he was bullied. He'd fight with people all the time, never pay attention in class; before long, his nickname was 'trouble'.

It's possible that Gosling, intentionally or not, is the most 'Method' of young actors, harnessing the rebellions and contradictions and exclusions of his youth to portray characters who are engaged in battles against themselves. The teacher he plays in Half Nelson thinks he is struggling to save inner-city teenagers from a life of drug pushing, yet the example he sets is far worse. As one of his own students enters a seedy, prostitute-filled motel room to hand him a small package, the question of whether anyone can save anyone else in this situation is left open.

To research the role, Gosling shadowed a history teacher in Brooklyn. That research was vital because when Gosling was 10 his mother took him out of class and home-schooled him. The year spent with her, watching movies, listening to Chet Baker and Billie Holiday, gave him, he says, 'a sense of autonomy that I've never really lost'. That confidence was crucial: 'I didn't want to work in a paper mill, and I wasn't going to stay in school. I hated being a kid. I didn't like being told what to do, I didn't like my body, I didn't like any of it. Being a kid and playing and all that stuff just drove me nuts.'

So when The Mickey Mouse Club auditioned children in Montreal, 12 year-old Ryan went along. He was hired by Disney, and went on to do an indelible stint as a 'Mouseketeer' with Britney Spears, Christina Aguilera and Justin Timberlake, at which point he allegedly proved such a bad influence on the girls that their mothers complained to Disney. According to Gosling, 'I just told them what I heard - like positions and stuff.' Teaching Britney Spears about sex: Now there's an achievement! 'I feel somewhat responsible for how sexual she is now,' he said when Britney was at the height of her fame, 'When I see her with a snake around her neck, I think: Did I do that?'

At 16 Gosling moved to Los Angeles for good, and sought out the closest thing to reality. 'There's something wrong when you have a grown man calling a kid "Sir",' he says of the child actor's life. 'That fucks with your head.' Instead of living in Hollywood or Beverly Hills, he has found a home in downtown LA for himself and his canine alter-ego George ('He looks like a little guy in a suit, and he doesn't like it that he's a dog, you can tell'.). There, he says, people 'are all doing something different. They don't all have a script in their car. I live on Skid Row. You can't filter yourself from reality there.'

He likes to work - not just acting work, but manual things. He and a friend have opened a Moroccan restaurant called Tagine in Beverly Hills, and Gosling can often be found in the kitchen. Two summers ago he worked in a local convenience store run by an Iranian who is now a friend. He stacked shelves, made sandwiches. 'It was fun because I had a job where homeless people could tell me what to do.'

For the past couple of years he has had a long-distance relationship with his Notebook co-star and fellow Canadian, Rachel McAdams, who lives in Toronto. No one was more surprised to hear of this than the people who'd known them on set. 'They hated each other. And now they're dating?' said the film's director, Nick Cassavetes. Once, during filming, Gosling asked if he could have a stand-in for his reaction shot because he felt she was so uncooperative he couldn't bear to look at her. Cassavetes refused. A screaming match ensued between the co-stars.

Gosling doesn't deny this. 'We inspired the worst in each other. It was a strange experience, making a love story and not getting along with your co-star in any way.'

And then what? I ask.

'I don't know what happened ,' he says. 'Two years later I saw her in New York and we started getting the idea that maybe we were wrong about each other ...'

The week after we meet, Gosling is planning to return to Africa to research his latest project. A couple of years ago, at a time when most journalists were too afraid to set foot there, Gosling visited refugee camps on the border between Darfur and Chad. 'You ask yourself what you can do,' he reflects. 'The only thing I know how to do is make movies.' He has now written a film called The Lord's Resistance, about child soldiers in Uganda, and is looking for children who have had those experiences to play the roles; the film, he hopes, will be like a partially scripted documentary, or a feature film with a documentary effect. He is unassumingly valiant about all this, smirking as he anticipates accusations of an Angelina Jolie syndrome. Having a social conscience 'seems like it's becoming popular in Hollywood,' he shrugs. 'As embarrassing as that is, it's great. Celebrities' intentions, like a lot of people's, aren't pure. But they're doing something.'

Gosling is both intense and laidback, strong-willed yet oddly prepared to throw it all up in the air. With a beatific smile he tells me he aspires to the condition of 'old man': Free to do as he pleases, grumpy and lascivious and devil-may-care. 'He's like one of those freaks,' Cassavetes said a few years ago, 'he's 23, but he might as well be 63.' You get the impression that he feels about life the way he relates to other things he's saddled with. At one point, glancing at his hand, I see what I think must be a fading ink stamp, and ask if he was at a club the night before. 'Oh no,' he laughs, 'that's a homemade tattoo'. He has many, apparently, all of them 'ridiculous'. 'A tattoo should never be meaningful,' he explains, 'because at a certain point you're going to hate it, and it might as well make you laugh.'

· Half Nelson opens in April