One of the central themes of the movie Black Swan was the rivalry between Natalie Portman's character, Nina, and her understudy, Lily, played by Mila Kunis. As Nina struggled to embody the role of the dark, seductive black swan in addition to the pure, innocent white one, Lily threatened to steal the part from her.Portman recently revealed in a Vogue interview that this was also happening in real life. Director Dareen Aronofsky would say, "Oh, Mila is doing really well on her stuff. She’s so much better than you," she said. "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." Aronofsky has denied this but did admit to separating the actresses.All in all, Black Swan doesn't sound like it was a fun movie for Portman to film. On top of this contentious relationship with her co-star, she also had to train to perform in the ballet that takes place within the movie. During her 16-hour-a-day trainings, she dislocated her rib.Portman definitely deserved that Oscar she won ( and then misplaced ) for Black Swan. And thankfully, she and Kunis's friendship has survived.