The actress takes a serious turn in the dark crime film, in which she plays the girlfriend of a psychotic killer, played by Affleck.
But she found it difficult to make one graphic scene, in which she is spanked while lying semi-naked on a bed, seem convincing.
She explains, "There were a couple (of spankings) in there when I thought, 'God, Casey!' He got a bit of power behind it. It was definitely real.
"It depends on your sense of humour where you draw your boundaries. After about the 20th spank we all started to laugh because you become quite comfortable with the idea and it becomes about the technical side of things.
"I've known Casey for so long that you can't help but laugh every now and then. I'm pretty comfortable with my body. I'm imperfect. The imperfections are there. People are going to see them, but I take the view you only live once, and, hey, I'm getting spanked today!"