Miller struggled with G.I. Joe role

Sienna Miller is apprehensive about the release of her upcoming movie G.I. Joe - because her performance is "not the best acting" she's done.

The Brit star plays a leather-clad spy in the film - and insists it was a challenge to move from dramas such as Factory Girl and The Edge Of Love to an all-action role.

She explains, "G.I. Joe is not going to be the best acting work I've ever done.

"The guys I'm fighting are all really tall - and I'm not. I have to kick a lot of them in the head, so just learning to kick very high was a challenge. Looking like you're a good fighter is hard."

And the 27 year old admits her performance was hindered by her costume - a black leather catsuit.

Miller adds, "I won't be wearing one again. Squeezing myself into that with the aid of talcum powder every day for five months was more than enough. I could barely move in it."

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