Sienna Miller hams it up as the Baroness in G.I. Joe

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Dec 14, 2012, 3:54 PM EST

Sienna Miller, who plays the Baroness in the upcoming G.I. Joe: The Rise of Cobra, told a group of reporters that she had to learn to act in a more heightened style. "You have to commit to slight overacting and really performing in a different way," Miller said in a group interview Thursday in Beverly Hills, Calif., where she was promoting The Edge of Love.

Perhaps better known for her real-life love affairs with leading men such as Jude Law, on film Miller may be best known for dramas such as Alfie, Layer Cake and Factory Girl. All were intense dramas with few, if any, explosions. Her role as Victoria in Stardust was the one role without visual effects and spectacle.

In G.I. Joe mythology, the Baroness is the intelligence officer for the nefarious organization COBRA and lieutenant to Cobra Commander. Miller played the character with long black hair, glasses and a leather catsuit, quite a departure from her natural blond hair.

The Stephen Sommers-directed Joe is out Aug. 7, bringing the franchise of military action figures to the big screen. The following Q&A features edited excerpts of the group interview with Miller.

What led you to choose G.I. Joe after films like Factory Girl and The Edge of Love?

Miller: I had never done a huge studio movie like G.I. Joe, and I felt like it was time to do something where I wasn't having a breakdown or addicted to heroin or dying at the end, something that was just maybe really great fun and that people went to see and actually just had a great time seeing and weren't left damaged. I think my parents also [felt] like, "Please, just stop doing these films. You're killing us."

And how did the experience measure up?

Miller: It was another different world. It's a huge budget, a huge crew, things blowing up around you all the time. I think initially I was just completely overwhelmed. I'd never done fight training, let alone been to a gym, so this was like a whole new experience. Then, once I kind of realized what I was doing, it was really fun to make a movie that is just pure entertainment and a completely different style of acting. I think the variety of doing a film like this to doing something like G.I. Joe is what's really exciting. Next I'm doing a play on Broadway.

What was the most fun about G.I. Joe?

Miller: Learning how to fight. A little bit of exercise, which is not something I'm [usually into].

Did you have G.I. Joe in England growing up?

Miller: We called him Action Man. It's so British, Action Man.

So did you know about the Baroness before doing the movie?

Miller: No, not much, but I know an awful lot now [laughs].

How do you think you look in the Baroness costume?

Miller: Different. Unrecognizable, I think, which is [not a] bad thing.