In the film Portman plays a ballet dancer who's so anally retentive that she'd make me look positively chilled, were she real. She gets cast as the lead in Swan Lake, where she plays "uptight" and "loosey goosey" swans who want to seduce the same man. Unsurprisingly, she's got the "uptight swan" nailed, but can't get the "loosey goosey" element at all, and has a nervous breakdown trying to get it.
The build up to her breakdown isn't bad if you accept that Portman's character will start bouncing round the ceiling under the stress, but I'm not convinced that she would. I can't believe anyone could go that far off the deep end over what is effectively a job.