She and Natalie Portman play rival ballet dancers in Darren Aronofsky's Black Swan. Kunis, who was cast in the film based on her performance in Forgetting Sarah Marshall, and on the recommendation of costar Natalie Portman, underwent a training regimen that included cardiovascular exercise, a 1200-calorie a day diet (she lost twenty pounds that she regained after filming ended), and ballet classes for four hours a day, seven days a week. The film premiered as the opening film at the 67th Venice International Film Festival on September 1, 2010 and had a premium screening at the 35th Toronto International Film Festival on September 13. Its initial limited release in select theaters took place on December 3, 2010. Early reviews of Kunis's performance have been positive, with Kirk Honeycutt of The Hollywood Reporter stating, "Kunis makes a perfect alternate to Portman, equally as lithe and dark but a smirk of self-assurance in place of Portman's wide-eyed fearfulness." Guy Lodge of In Contention praises Kunis saying, "it’s the cool, throaty-voiced Kunis who is the surprise package here, intelligently watching and reflecting her co-star in such a manner that we’re as uncertain as Nina of her ingenuousness." Kunis's performance won her the Marcello Mastroianni Award for Best Young Actor or Actress at the 67th Venice International Film Festival.
Kunis was cast alongside Justin Timberlake in the romantic comedy Friends with Benefits, which filmed from July to September 2010 in New York City and Los Angeles. Director Will Gluck stated that he wrote the story with Kunis and Timberlake in mind. "There were a couple of actors I wanted to work with, so I wrote it for Justin Timberlake and Mila Kunis. I wanted to do more of an adult movie about sex, too, and about relationships."
