Once in a while an actor makes such a colossal impression it's enough to make an entire, cynical industry sit up pay rapt attention. Michael Fassbender had just such a cosmic breakthrough in artist Steve McQueen's Hunger, in which the German-born Irish actor blew hearts and minds as IRA figurehead Bobby Sands, who lead the fatal 1981 hunger strike at the Maze prison over the criminal (rather than political) status of Republican prisoners. It wasn't merely that he emaciated himself to an agonizing extent to play Sands.

Last year, Fassbender took a further leap, consolidating his newfound status as the smooth-talking council-astate opportunist who seduces a 15-year-old Mia (Katie Jarvis) in Andrea Arnold's Fish Tank, and as the undercover British who gets his testicles shot off during an insane shootout in Tarantino's WW2 pastiche Inglourious Basterds. He might describe his method for sinking into a character as "sheer, boring repetition, just reading the script, reading the script, reading the script," but Fassbender adores the shape-shifting nature of his profession.

At the Wonderland photo shoot in north-west London, he sheds the outfit the arrives in and quick-changes into various garments for the shoots like a man on an especially pressing mission. If on-screen he appears ruggedly, Teutonically handsome, in person he's even more striking, his granite jawline more defined, his aquamarine eyes more piercing – and twinkling with easy Irish charm. His physique would make an Olympic gymnast proud – his lean, sculpted frame doesn't appear to carry an ounce of fat. "I've always done a little bit of sport so I have never been to the gym in my life," he says, modestly.

Grabbing quick smokes and conversational snippets between set-ups, he's languid, affable, tapping me on the leg or winking in apology whenever he's beckoned back to the studio floor, but unable to switch from distraction to concentration until the shoot's wrapped. "Sorry man," he says, "it's be easier now to give you my full attention..."

Fassbender was two when his family left behind an industrial pocket of Germany for the green, wide-open spaces of Killarney in south-west Ireland, where his parents still rub a restraint. Fassbender endured "plenty of slagging" over his German roots although Killarney was still "a great place to grow up as a kid. It's not like being in the city where there's lots of bad things to temp you – the most danger we came other over the head with a stick. And we loved right on the lakes so I used to go fishing all the time."

Unlike so many of his peers, Fassbender didn't display a precocious urge to perform from the moment he could talk; he didn't put on little shows for relatives, or attend youth theatre from the age of five. He was just a normal working-class Irish lad who tore about with his friends and scrubbed pots in his parents' restaurant – until he reached 17, and realized he had no clue what to do with his life. "I thought maybe a lawyer would be interesting," he says, "but I'm a slow reader so that volume of reading wasn't a good idea."

When he was lured into acting workshops, he found he had a penchant for it and, two years later, headed off to study at the Drama Centre in London – although he never passed the finish line as he landed an agent and dropped out in his final year. "I'm a quitter," he jokes. "No, I just decided it was time to go. My year was starting to implode on itself. It was an intense place and it could breed a bitchiness and animosity, so I decided to get out of there – before the agent show came and it got really ugly. Which probably was a mistake because loads of casting directors just didn’t know who I was for years."

His acting career started promisingly enough when he was hired to play one of the American grunts in the Spielberg/Hanks-produced WW2 series Band of Brothers, only to be out of work for a year after it finished. "I thought to myself, 'What else can I do? I never went to university, I don't have many things to fall back on, so I'll just learn as much as I can about working behind a bar and making drinks," he muses. "I thought one day I could run a bar if it doesn't work out."

In the two years leading up to Hunger, however, Fassbender's career gathered a mild head of steam. He snatched the role of Romola Garai's paramour in Francois Ozon's OTT melodrama Angel from under the noses of bigger names; he starred as the fallen angel Azazeal in Sky One's teen supernatural series Hex; and he was one of Gerard Butler's six-pack warriors in 300, which turned into a massive global hit. "I embraced the camp," Fassbender laughs of the muscle-toned hunkfest. "What else are you going to do? You're running around in leather Speedos."

Then came Fassbender's career changer. "Hunger allowed me to vie for leading roles, to be taken seriously as a leading man," he observes. "It was the perfect film to allow me to show what I was capable of doing. And it came at just the right time." He underwent a dramatic 10-week transformation to play Sands, holing up in an LA apartment – reasoning blue skies and sunshine could help keep hunger-induced depression at bay – and surviving on 600 calories a day in the home stretch before calling it quits at 59 kilos, which is right before organ failure can kick in. But Fassbender insists the weight loss was never about showing off or actorly indulgence. "It was just a given it had to happen because the man starved himself to death," he says. "And for me the difficult part was the speech with the priest – that was the crux of the piece. If we made a mess of that then the whole film would have fallen apart. The weight loss I just went through by numbers – it was just a calorie count. But I was hungry..."

Thirty when he made Hunger, Fassbender's not kidding when he suggests it came at the right time. In a competitive, seething field of talented actors, he knows he's one of the fortunate few. "You need a few guardian angels to help you along the way because it is fraught with so much lick and timing," the actor admits. On the heels of Fish Tank and Basterds, Fassbender shot Neil Marshall's Centurion, in which he plays a Roman soldier trying to brave the savage wilds – and wild savage – of Scotland. "It was running through two feet of show up Scottish mountainsides, semi-clad. Noel Clark got frostbite on his toes on day two. We were out there with the elements, but I loved it."

He also landed his first Hollywood villain, "a nasty piece of work" in the zombie western Jonah Hex, which he shot last spring before decamping to LA (where his sister live) for the latter half of 2009, chilling, hanging out, reading scripts and making his first appearance at geek-convention Comic Con for Jonah Hex. "Every second question to Megan was, 'Do you have a boyfriend? I'm available.' That must be frustrating for her to just be seen as this object of sexual desire," says Fassbender, adding with a laugh, "Which is not a problem for me."

Hanging in LA also afforded him the opportunity to "take a breather, have a moment to get everything into perspective and figure out what I wanted to do next. I've been knocking around for a while and it's nice not to have to take everything that comes your way and, you know, try and make lemonade out of lemons all the time. It's about getting inspired now, about finding things that hit you in the gut."

And if that includes working with some of Hollywood's biggest names, all the better. Fassbender's ambitions go beyond making hay while the sun shines. "I feel supremely blessed to be working now with people like Quentin and Steven Soderbergh. It's all down to the people you work with and they bring out the best in you. You know you're in good hands and you can really put your trust in these directors, so they end up taking you that extra few steps. It's been unbelievable and, yeah, I hope I'm as lucky in future projects to be working with that calibre of director."

Back in Europe until late summer ("I really missed Europe"), he's about to start shooting Jane Eyre, starring as Mr. Rochester opposite Mia Wasikowska, followed by David Cronenberg's The Talking Cure, an adaptation of Christopher Hampton's play in which he'll play psych-legend Carl Jung to Viggo Mortensen's Sigmund Freud. "It's a psychological drama with a thriller edge to it," says Fassbender, who's been grilling his sister – a neuropsychologist and Jungian expert – for insight. "He was definitely less of a conventional thinker than Freud – a very experimental character. I've got my work cut out."

He's thrilled about the prospect of working with Mortensen ("his performance in The Road was just blinding") and has bonded with Cronenberg over their shared passion for motorsports. "He's a big Ducati fan," whereas Fassbender keeps it British – he's the proud owner of a Triumph Speed Triple. It needs a new clutch, but it's the perfect accessory for an actor who spent so many years in the wilderness grinding his gears before finally finding the right conduit, and whose sudden burst of velocity is whisking him from boy's own adventures to Bronte romances to psychological chillers. Like I said, he's a man on a mission. "I like to keep scaring myself, I like to explore human beings, I like to jump into all these different clothes and get under their skins," he says with conviction. "And then I'm ready to move onto the next one."


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