By JULIE BLOOM

Ten years of serious training and then five more toiling in the ranks. That’s how many years of dedicated study it takes on average to become a principal ballerina at a top company. But Hollywood isn’t willing to wait.

“It’s not the same as Mickey becoming a wrestler because that’s a craft you can learn in a few months,” the director Darren Aronofsky said, referring to Mickey Rourke, who starred in his film “The Wrestler.” “Ballet is something you have to be trained from a tiny age.” Mr. Aronofsky’s latest movie, a rumored Oscar contender, “Black Swan,” due for release in most countries this year and next, is a psychological thriller centered on a fictional ballet company’s new version of “Swan Lake.” Natalie Portman plays the lead ballerina .

“At the beginning it was a big question because we didn’t know if any actor could pull it off,” Mr. Aronofsky said of the role of Nina, who turns into the Swan Queen onstage. When Ms. Portman, 29, took the part, she said, “I really thought I was better than I was.” She wasn’t completely new to ballet, having studied as a child, but at 13 she had traded in her slippers to act.

“It was a rude awakening to get there, and to be, like, I don’t know what I’m doing,” Ms. Portman said . In George Nolfi’s “Adjustment Bureau,” to be released next year, Emily Blunt stars as a member of a real troupe, Cedar Lake Contemporary Ballet. Ms. Blunt, 27, who studied with Benoit-Swan Pouffer, artistic director of Cedar Lake, for her role, felt pressure to learn as much as she could .

“These dancers don’t want some actor misrepresenting the brilliance of what they do,” she said. The effort to avoid that consumed Ms. Portman. In the film Ms. Portman does indeed dance, about 10 sequences, with a lot of work for her upper body. The difficult point work and turns were performed by a body double, Sarah Lane, the American Ballet Theater soloist.

To prepare her body, Ms. Portman started more than a year in advance. Mary Helen Bowers, a former City Ballet dancer from North Carolina, combined basic ballet technique and exercises to make Ms. Portman’s physique more like a dancer’s, with the sinewy, lean muscles, upright carriage, pressed-down shoulders and tell-tale elongated neck.

“We thought that was as important as being able to move,” Ms. Bowers said. Wherever Ms. Portman’s career took her, she trained at least five hours a day with Ms. Bowers. Perfecting something as seemingly simple as the undulating swan arms was one of her greatest struggles.

She practiced for hours and watched YouTube clips of famous swan queens like Alicia Alonso and Natalia Makarova. The physical extremes of the art form were what most interested Ms. Portman and Mr. Aronofsky. “The contrast between what you see onstage and what is underneath is part of the resonance of this film,” Ms. Portman said.

“That it’s supposed to look easy and painless and carefree and light and delicate and just pretty, and underneath it’s, like, really gruesome.”