The 29-year-old stars alongside Kunis as a ballet dancer who slowly loses her mind in Darren Aronofsky's new thriller Black Swan.
And Natalie found it hard balancing both the emotional and physical demands of the role.
She told Entertainment Tonight: 'To do something that's really physically challenging for yourself and also have to act at the same time was hard because sometimes you have conflicting needs.
'Like sometimes you need to have a real confident, go-getter attitude to nail your turn, and then you need to be insecure in the scene, and those demands can clash.'
Before Portman had to act in a lesbian sex scene with co-star Kunis, 27, the pair were friends, which made it more difficult.
She said: 'It was definitely awkward.
'Darren was helpful in just saying, 'You know, if you guys just commit to it, then you're not going to have to do it again. Just do it, get it done. If you're shy and try and be giggly and embarrassed all day, you're going to have to do it over and over and over.''
The Hollywood star added: 'That was a good way of looking at it.'