How Darren Aronofsky transformed ballet swan lake into the thriller black swan

In a thriller you expect to see zombies, vampires or runaway mental patients but no one expected this one. The new movie Black Swan is a thriller about a ballerina. The movie stars Natalie Portman as a mentally deranged dancer so obsessed with her starring role in Swan Lake that she begins an excruciating physical transformation into a black feathered beast. Darren thought about the ballerina turning into a beautiful swan and thought, "i could make this gruesome"