In Shame, Mulligan plays a young woman who helplessly watches as her sex addict brother's (Fassbender) life falls apart.
In an interview with USA Today, Mulligan revealed that Fassbender went quite far to get inside the mind of his dark character.
"Michael is genial and great, and doesn't put on a personality for people. He just is who he is," Mulligan said. "[While filming] I saw the danger in him. Whether it was a violent, aggressive scene or a calm and empathetic one, I saw danger there."
Mulligan continued: "More often than not, he would be different in every take. He had the license to grab me and do things. You can't be delicate and careful. It was so exciting, I never knew what he was going to do."
Fassbender appeared more light-hearted when asked about his reputation as a brooding actor.
"I'm dark and dangerous all the time," he laughed. "I don't know. I've been lucky. This is what I dreamed of when I started at 17.
"All my heroes like John Cazale, Robert De Niro, Al Pacino, Brando - there was an element of the character actor in these guys."
Shame is now playing in the US and opens January 13, 2012 in the UK.
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Watch a trailer for Shame below: