Aronofsky was determined to make the onscreen tensions as real as possible, so he kept the two stars away from each other during filming.
He tells the Los Angeles Times, "I knew it might be really hard to keep them apart because they're friends, but I just didn't want them to know each other's motives. I didn't want them to compare notes. I wanted them to come from different places."
Portman reveals the director tried hard to create a rivalry between the pair as they both trained hard to perfect their ballet moves.
She adds, "He'd say, 'Oh, Mila is doing really well on her stuff. She's so much better than you.' Darren would tell us things about each other to try to make us jealous. I think he was trying to create a rivalry in real life between us."