The Star Wars beauty was determined to pull off the tricky ballet steps herself to play Nina Sayers in the Darren Aronofsky film, and spent up to eight hours a day in training to achieve the physique needed to make her portrayal believable.
She tells Entertainment Weekly, "So many of the emotional scenes happen while she's dancing that there was no real way of getting around it.
"I do have a double for the complicated turning stuff. But anything I could do myself saved the budget hundreds of thousands of dollars in special effects."
And Portman managed to remain disciplined while shooting upcoming fantasy comedy Your Highness with James Franco and Danny McBride in Ireland.
She adds, "(They) were out having fun every night, and I was the little good girl; no drinking, waking up at five to do my workouts, and not eating. I was the really unfun one."