Darren Aronofsky told journalist Kim Masters in a radio interview (KCRW's "The Business" broadcast February 14, 2011) that Natalie Portman not only trained for a year as a dancer to prepare for the role, but paid for the training out of her own pocket until the film found investors. Aronofsky attributed the film getting made at all to Portman's dedication and enthusiasm. She even accepted a role in Your Highness (2011) mainly for the money in case she needed to help co-finance Black Swan.
Mila Kunis was brought into the project after co-star Natalie Portman suggested her to director Darren Aronofsky. Kunis had a video chat with Aronofsky via Skype and got the role without officially auditioning.
Natalie Portman revealed that director Darren Aronofsky would subtly try to pit her and Mila Kunis against each other during filming in an attempt to increase the on-screen tension between their characters. This included keeping the two actresses separated during filming and sending each of them intimidating text messages about each others performance that day. However, according to Kunis, this backfired; because they were already good friends before filming, whenever they would get wind that the other one was doing really well, they would respond in congratulatory support, not rivalry.
When shooting Black Swan, Natalie Portman sustained many injuries, which caused delays to the production. The most common injury was her left toe, which had to be restrapped while the cast and crew waited. This kind of delay was so frequent it was called a "Natalie Portman's Left Toe Delay" which was later shortened to...Portmantoe.
Darren Aronofsky: [Perfect Blue] (at around 51 mins) The overhead shot of Nina in the bathtub is an exact replica of a shot in the Japanese anime thriller Perfect Blue (1997). Black Swan itself contains numerous similarities, both in narrative and style, to Perfect Blue, though Aronofsky has denied that it was an influence.
Darren Aronofsky: [following] (at around 5 mins) The camera follows Nina from behind when she leaves the subway and goes to the ballet studio.