Quentin Tarantino's Inglourious Basterds (opening August 21st) places a cadre of Jewish-American soldiers deep in the heart of Nazi-occupied France, where they use brutal tactics to put a scare into the goose-stepping thugs. Leading the charge baseball bat-first is Sgt. Donny Donowitz, played by writer-director...and now actor?…Eli Roth.

So how does a guy go from directing horror movies to starring in World War II epics with Brad Pitt?
Quentin Tarantino could answer that better than I. Let's just say, I didn't go the normal route one does that. It takes someone like Quentin Tarantino to see someone's full potential. Sometimes you need a nudge from someone you really trust. And when someone has faith in you it makes you feel like you can do anything. Quentin was so encouraging about my acting, right from when he saw me in Cabin Fever and that was just me replacing an actor at the last minute. I never planned on being in the film. It was one walk-on scene. From there he put me in Death Proof, and in Death Proof he was so happy with my performance in that one scene in the bar that he wanted to put me in Inglourious Basterds. He said, "You have a potential here that you are not mining. You have real acting talent and you have screen presence and you get the dialogue, you understand the story beats, you know how to play to the camera." I had studied acting and directed actors, but I never had a reason to fully push myself. All I ever wanted to do was direct. But I enjoyed acting. So then Quentin says, "You're going to be in Inglourious Basterds. And you're going to be onscreen with Brad Pitt." So I thought, "f I'm going to do this I have to push myself harder than I've ever pushed myself in anything in my life." Because if it doesn't work, people are going to know right away. So I thought, "I have to be like Robert De Niro, where he throws himself into a role." I've always wanted to do that but I never had a reason to do that. So I put on 40 pounds of muscle and I went back to Boston where I'm from and I only talked in my accent and I researched the character.

Did you work out a backstory for Donny?
Everything. Quentin said, "You have to know this guy the way you know your best friend. The way he'd react if a girl walks in, a dog walks in, who his parents are…

Do you remember some of the specific elements?
Yeah, we filmed several sequences of me in Boston cutting hair. I trained to cut hair. And I practiced. Because there was a scene where I had to talk and cut hair and it had to look effortless. Then you see me getting the bat and going to see this little old lady in the neighborhood, this Jewish woman, played by Cloris Leachman. I have her write the name of someone she's worried about on the bat. The scenes were great. But Quentin said it was taking away from my entrance. He's got this whole prequel written and he said if he gets to shoot that prequel, now he has three scenes shot for it.

So he may actually still use that stuff?
He's not putting them on the DVD specifically because he may be using them.

So how else did your De Niro method help you?
I told Quentin, "Look, I'm going to Berlin and I'm going to be there for 6 months. I'm not leaving. I'm not going to fly back and forth to L.A. I'm there for the whole time. So if you need help [getting the film finished in time for] the Cannes Film Festival, give me a second camera. I'll help you pick up shots you don't get." And he said, "Well, I've never done that, but there's this whole other movie I'm suppose to shoot called Nation's Pride (a propaganda film produced by Joseph Goebbels in the movie) and I can't even deal with that because it's taking everything I have to make Basterds. I'd love for you to do that. It was like my [Thanksgiving] trailer for Grindhouse—in, like, 2 days I gave him 130 shots. I flew out my brother Gabriel and I knew how to do it—after making three films and the Grindhouse trailer, I knew how to shoot a fake movie fast. Quentin was amazed. And I felt like I had given him another character for the film. And I knew people were going to grade me on an extra hard curve in the movie. They know I'm friends with Quentin and they know me as a director. So they were going to be like, "Why did he cast him? Is it just because they're friends?" So I knew I had to win the audience over. I couldn't just do that by bulking up. Anyone could do that. That's easy. What's going to sell it is the look in that character's eyes. When you see him with that bat, he has to look like he's ready to kill. And you have to feel that pain and that rage and that fire. That he just wants to beat every Nazi to death with his hands. And that took everything I had to work myself into that state and I would never had done it if Quentin had not pushed me.

Quentin loves to tie his movie universes together, so we have to ask: Is your character, Donny Donowitz, a distant relative of movie producer Lee Donowitz from True Romance?
No, they are not actually relatives. But "Donowitz" is someone from Quentin's life, so that's why he likes that name.

This was a big international set, was it hard having to bullshit between takes in four different languages?
Well, I had that in Hostel, so I had experience with that. It was great. European actors are amazing. They're so good. It was wonderful to work with them. They have—well, everyone on the set had—a really great work ethic. And they just love doing it. You could just feel that everyone was so happy to be in a Tarantino film. We were all enjoying each other's performances. You could feel that in the first read-through. The first day we all got to Germany Quentin had everyone, the whole cast, get together and read through the script. And you could feel it in the room.

Has working on this film changed what you have planned next?
It hasn't changed what I plan to do next, but I'll tell you what it has changed. Quentin said, "Eli you have another career here if you want it. If you want to be a movie star, you could." And I always said, "Well, that's up to the public. Besides, why would I ever act for another director, how could it get better than this?" And he said, "It might not, but now you can write great parts for yourself and no one will give you shit for it." And that really opened me up creatively. It made me think, "Well, here's this idea that I never thought I'd be able to do. I can write that for myself." So I started to think about those ideas. Not necessarily to do next, but somewhere down the road. I feel like I can do anything now.

Now Donny's nickname among the Nazis is "The Bear Jew." Are you prepared to have that shouted at you in the streets?
The things that I've been called in the past when people see me on the street? Believe me, "Bear Jew" is an upgrade. I've always been associated with some furry animal. In high school I found out that there were girls who were calling me "Wolfy." And then in Prague one of the extras saw my chest hair and called me "Gorilka" which is Slovak for "Gorilla," so all the guys in Hostel were like, "Gorilka!" It was inescapable, I was Gorilka. Which evolved into "monkey," which was a nickname from a girlfriend. Now it's "Bear." I'm quite used to it by now.