The 29-year-old Leon star told The New York Times: 'I'd studied ballet from the age of four to about 13, at the American Dance Theatre Workshop on Long Island, which is associated with the Eglevsky Ballet, and from the time I became an actress, I think I always wanted to do a ballet film.'
Portman told the US newspaper that shooting the flick was so intense that she ended up getting very little sleep.
She said: 'Basically, I didn't do anything except work, there was no, like, meeting up with friends for dinner or going to the movies.
'We'd do 16-hour days, then I'd go home and work out, because I had to stay in shape, and I’d prepare for the next day’s scenes and then get maybe five hours of sleep – it was really, really extreme.'
The Star Wars star added: 'It was scary, disciplined and rigorous, and I think, in a way, that fit into the mindset of the characters.'