The Book Of Eli actress plays rival ballet dancer Lily to Natalie's Nina in Black Swan, and emotions between the duo come to a head during a graphic sex scene.
"Anytime you do any intimate scene on film, it's going to be a little uncomfortable, whether it's the same sex or the opposite sex," she told Collider.
She credited her close friendship with Natalie for making it easier.
"The truth of the matter is that we were friends before we started the film, so that made it a lot easier," she continued.
"We didn't really discuss it very much. We just kind of did it. It made sense for the character. It wasn't put in for shock value. It wasn't something that we needed to justify in our heads, as to why we were doing it. That was it."
It was Mila's first sex scene with someone of the same sex.
"I did a film called After Sex with Zoe Saldana, where she played my girlfriend, but we never had a sex scene. We had the scene of what occurs after you have sex, so maybe that doesn't count. So, this would be my first," she added.
Black Swan opens in cinemas on January 21.
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