I wrote this when I first saw the movie months ago, but never got around to posting it.
If you ask people what happens in Black Swan, a lot of them will say a young woman goes from being nice and sweet to being completely unstable, but this is totally inaccurate. The movie begins with Nina awakening and telling her mother, who she still lives with, of how she dreamed about becoming the Black Swan in Swan Lake. Nina's relationship with her mother is bizzarre, her mother at one point forcibly removing Nina's clothing, supposedly to investigate scratches Nina gave herself and cutting her nails to prevent her from scratching herself again. The first 15 minutes make it clear that Nina is already a very screwed up person on a lot of levels.
Nina has no friends at the ballet company. At several points, she is rude to her fellow dancers, partly because she's shy, and partly because she's an asshole; she's so focused on her role that she doesn't really care about anything else. The two nonfamilial relationships she does has are with Thomas, the director of Swan Lake, and Lily, a new dancer at the company. The reason for her relationship with Thomas is obvious, and he's trying to seduce her, though they never go further than making out. The reason for her relationship with Lily isn't as clear; she treats Lily the same way she treats all her other fellow dancers, with a mix of indifference and rudeness. But for some reason, Lily tries to befriend her anyway, asking Nina out for drinks and offering her ectasy, which Nina eventually accepts. Later that night, Nina hallucinates that Lily she and Lily have an affair.
Even though Lily is one of the closest "friends" Nina has, if this were a normal movie, Lily's role would be considered a small one; for one of the main characters, she gets very little screen time. And even though Nina imagines that she and Lily have sex, she apparently doesn't have feelings for Lily, as she continues to treat Lily the same way she treats the other dancers: crappily. I'm beginning to think the only reason for the sex scene, no, the entire movie, was because the producers really wanted to film Mila Kunis and Natalie Portman going at it. Which is not necessarily a bad reason to make a movie.