|Christian Bale, Amy Adams and Bradley Cooper in "American Hustle"|
The actress had to punch her co-star during a scene in the new 70s thriller but initially wasn't doing it hard enough and had to be encouraged by director David O. Russell.
She told ABC News: "I believe you can accomplish something without hurting someone, so I kept hitting him and I guess it wasn't reading as hard enough. So David's yelling, 'Hit him, hit him', and Bradley's yelling, 'Hit me, hit me', So I did. And his face started swelling.
"It really started swelling in real life and I said, 'I certainly hope you have it [the shot] because I think Bradley needs some ice. I felt very badly about that."
Amy, 39, is also seen in a number of glamorous period outfits during the movie, but says numerous costume changes were a hassle.
She added: "I had to change costumes like 50 times. My character is trying on all of these different costumes and she's trying all of these different personas, so the costumes were a big part of how she defines herself, it's like her armor.
"It seemed to be, the stronger she feels the less clothes she wears. She uses it as a manipulation and as a weapon. But it was such an insane shoot, a lot of the costume changes were, like, in a closet at some warehouse. You'd just find anywhere you could change and maintain a little dignity and modesty."