Cooper's co-star on the film Eric Closs told E! News the Oscar-nominated star had "put on like 40 pounds" for his role in the Clint Eastwood-directed drama.
"He's really been working hard. He looks great. He's really taken on this character," said Close, who plays a Defense Intelligence Agency agent in the film. "He really takes this story personally and wants to do an honorable job for Chris' memory and for his family."
"American Sniper" is about a Navy SEAL who "recounts his military career, which includes more than 150 confirmed kills."
The film also stars Sienna Miller, Luke Grimes and Kyle Gallner and is expected in theaters sometime in 2015.
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