Q: Although you have a German surname, you actually grew up in Ireland.

A: My father is German — he's a chef, with his own restaurant in Killarney — and my mother is from Northern Ireland. My father is an artist in the kitchen — French bistro food, done very simply — and my mother runs the front. She always had this desire toward the arts; she loves the movies. I think the fact that my father's last name was Fassbender, like the director, may have lent a hand in why she married him. [Laughs] And my father probably told her that he was related to the director, even though it's not true. [Laughs again]

Q: From a young age did you want to be an actor?

A: Not until I was 17. Then I wanted to be in comedy — I loved the movie "Fletch." I knew it by heart. Chevy Chase was a big hero of mine. I liked theater too, but movies enchanted me. My mother loved '70s American cinema. Her favorite actor was John Cazale. He died young, but he appeared in masterpieces. In "The Godfather," he played Fredo, the most thankless sort of role, and he was magnificent. When "Reservoir Dogs" came out, it was a huge inspiration to me. My friends and I decided to mount a stage production. I played Mr. Pink, the Steve Buscemi character. But I played it differently — more like Robert De Niro’s character, Johnny Boy, in "Mean Streets." My parents started to take me seriously as an actor. I worked in their restaurant during the day, and then I’d go do the play at night. They became more and more supportive.

Q: Did you go to drama school?

A: When I was auditioning for drama schools in London, someone told me that you know an actor when he walks into the room. I was very worried that they wouldn't say that about me. I did go to the Drama Center, but I dropped out before graduating. In drama school, they don't think of movies as a pure form like theater, and it's films that I love most. There's an intimacy in movies — I wanted to have the same impact on others that movies had on me.

Q: In "Hunger," which came out last year, you played the I.R.A. activist Bobby Sands, who led the 1981 hunger strike at Maze prison in Northern Ireland. He didn't eat for 66 days, and then he died. To play the part, you lost 40 pounds, reducing your body from slim to emaciated.

A: The weight loss was the least of it, in some ways. At first I was nervous about accepting the part. I didn't know whether an audience would want to see prisoners smearing excrement on the walls or suffering terribly. It was a challenge. I had to audition for the director [the artist Steve McQueen], and when I auditioned, I realized I had to play the part.

Q: How did you lose the weight? You're already quite thin.

A: We stopped production, and I had 10 weeks to lose the weight. At first, I ate only 1,000 calories a day. There's not a lot of fat on me to begin with, but after a while I wasn't losing weight on 1,000 calories a day. I had to go down to 600 calories a day for the last five weeks. I moved to Venice, Calif. I needed to be in a warm climate, away from people I know. I couldn't see anyone — I cut everyone off. But anything that would take up time in my day was a good thing. I'd speed-walk, do yoga, skip rope and then speed-walk again. I ate a lot of blueberries, some nuts and then, at the end of the day, a tin of sardines. It was brutal. When you stop eating, your mind changes — I’d look in the mirror and think I'd gained weight. I had no scales near me or else I'd weigh myself three times a day. I lost my libido completely. I realized that starvation could make you crazy: there's a thin line between control and madness.

Q: What was your first meal after "Hunger" wrapped?

A: Sushi. Steve McQueen and I had Japanese food at a restaurant in Belfast. I got very, very cold. My body wasn't used to handling large amounts of food. And then I gained the weight back in only two weeks. I'd been on strict regimens before. When I was in "300" [in which Fassbender played a Spartan soldier], we knew we were going to have to wear leather Speedos and very little else. So we had 10 weeks of training, a boot camp for six people. They let me eat whatever I wanted, but the workouts were brutal. We were battle-ready.

Q: "Hunger" was a sensation at the Cannes Film Festival in 2008, where it won the Camera d'Or for best first feature. Did you suspect that the film would be a sensation?

A: Well, I've taught myself not to anticipate too much. My first acting break was in "Band of Brothers" [a 2001 HBO miniseries that was executive produced in part by Tom Hanks and Steven Spielberg], and I thought, This is it. [Laughs] But I went back to London with my tail between my legs. When "Hunger" was in Cannes, we were underdogs. When we got a standing ovation, I turned to my dad and said, "It doesn't get any better than this." Now, because of "Hunger," I get taken to lunch instead of having to audition. [Laughs]

Q: But you auditioned for the part of Archie Hicox, the British film critic-turned-secret agent in Tarantino's "Inglourious Basterds."

A: I actually had prepared for another part, and Quentin said, "No, you are here for Hicox." I first read the part to sound like Michael Caine, but once I got the job, Quentin told me he wanted me to be like a young George Sanders. He had me watch all of George Sanders's films — "The Saint" and others. I studied everything Sanders did, and I found a lot of humor in the Hicox character. When I did my first scene with Brad Pitt, he just started laughing. Quentin has us verging on the totally ridiculous, but he somehow steeps the movie in reality. It's like an elastic band that he pulls and pulls until it almost breaks. But it doesn't.

Q: And you just finished filming "Jonah Hex" in New Orleans.

A: I'm the villain!

Q: Have you played a villain before?

A: Quite a few times. Sometimes in film, too. [Laughs]

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