"Even from the dream team of Steve McQueen and Michael Fassbender, a film about a sex addict isn't exactly everyone's cup of tea at first glance: it's such a sleazy-sounding, tacky-looking subject. Luckily "Shame" is anything but. The script is co-written by McQueen and Abi ("The Hour") Morgan: a powerful, gut-thump of a character study – of loneliness as much as sex addiction perhaps. Fassbender plays Brandon, an alpha New Yorker. To the outside world he's living the dream: slick (in truth pretty sterile) apartment, fantastic job, nice line in cashmere V-necks. But Brandon has a dirty secret. Sex. He wants it, needs it, non-stop: he hires prostitutes; can't get through the day without masturbating in the loos at work; his office computer is riddled with hardcore porn. There's not an ounce of pleasure in any of it. Then his sister (Carey Mulligan, peroxide blonde, ditching her trademark netball captain charm) shows up. She's the flipside to Brandon: needy, messy, neuroses hanging out for everyone to see.

This isn't in any way scientific, but "Shame" does seem to be a particularly difficult film for women to watch. That tickles Fassbender. He fixes me with a sarky grin and his laser blue eyes. "For women to watch? How do you think it was for me?" Fair enough. But, I venture, we're watching the film from between our fingers, seeing Brandon as an exaggeration of male sexuality. He does exactly what evolutionary psychologists tell us men are hard-wired to do: have sex with as many women as possible. Fassbender is not buying it. "I suppose that's what a lot of girls are thinking. But it's definitely not the scenario here." He reaches for a comparison with an alcoholic: "A lot of people have a social drink. But they don't have to wake up in the morning dry heaving, putting alcohol into their system just to function. Most men think about sex a lot. We know that. But for someone like Brandon it is a compulsion. That's where the shame comes in: you're no longer in control of your choices." He chuckles: "Don't worry, your boyfriend is not Brandon. I don't know, maybe he is?"

I'm taken with a statement McQueen made at the premiere of "Shame" in Venice: "It's difficult to be a human being right now. I wanted to show us as being fragile. This is not beautiful or pretty to look at. I just wanted to take the ostrich head out of the sand and let us have a look at ourselves." It sort of sums up the man and his movies: bolshy, provocative and exceedingly sincere.

"Brave" is an adjective that gets bandied about willy-nilly by critics to describe performances: but for once it's justified here. There was a seven-minute standing ovation when "Shame" premiered at the Venice Film Festival. Fassbender went on to win the award for Best Actor. But it must have looked like a risky character for an actor on the verge of becoming an A-lister. Isn't that what agents are for: to prevent their clients from making films that potentially sabotage their thinking-woman's-sex-symbol status? No, says Fassbender. He wouldn't work with people like that. Life's too short. "That's what's dangerous about this profession: people worrying about their image and their brand. That's not my job. I'm not out there to push brand Fassbender."

Born in Germany to a German dad and Northern Irish mum, he moved to Killarney when he was two. His parents, now retired, ran a restaurant, which is where the work ethic comes from. He moved to London, where he still lives, in Hackney, at the age of 17: it was around this time that he landed his first nude role: starring in an ad for a Swedish airline. "They told me I was going to be naked. I thought maybe I'd have some kind of... I don't know... apparatus..." He's cracking up. "Some pants or something." He says he finds the hoo-ha surrounding the sex scenes in "Shame" bizarre: 'Is it more normal to see me shoot someone in the head than have sex? I'm presuming you've had sex? Most people out there do. Maybe this is something that we can be adult enough or normal enough to discuss."