"It was just pasta and bread and pizza all the time," Portman told PEOPLE of her post-Swan diet at the film's Tuesday premiere at New York's Ziegfeld Theatre.
While filming the psychological thriller, though, a change of eating habits was key. "It wasn't really giving up [food]," Portman, 29, says. "It was just smaller portions … It was more quantity than a particular thing."
Another challenge: the grueling yearlong dance training it took to get into character.
"Working five to eight hours a day for a year … it's what real dancers live through," she says. "It was an amazing experience, but definitely a difficult one."
It didn't hurt that the actress, who has already nabbed a Film Independent Spirit Award nomination for the role, trained with choreographer Benjamin Millepied, whom she subsequently started dating.
"He knows how to change when things aren't working well," Portman says of their working relationship. "Sometimes I just couldn't do something. No matter how hard I worked, I couldn't do it. He would fix it to make it something that I could do."
Praising her beau's "creativity and intelligence," the actress says, "He was really flexible. Working with film and working with actors rather than dancers, that flexibility was key."