"At a certain point, I looked at [Natalie's] back, and she was so skinny and so cut," director Darren Aronofsky told "Access Hollywood." "I was like, 'Natalie, start eating.' I made sure she had a bunch of food in her trailer."
Meanwhile, famously slim Gwyneth Paltrow gained -- then quickly lost -- 20 pounds, and no one, besides Paltrow and her trainer, seemed to notice.
Actresses have often shifted their shapes for a role. But today's crop of waiflike actresses hardly have any weight to lose. And when they do gain weight for a part, the way Paltrow did, the additional weight barely registers.
At least when Renee Zellweger gained 20 pounds to play Bridget Jones, she looked like a normal woman or, as New York Times critic Stephen Holden put it, "utterly real."
But there's nothing real about the weight fluctuations of today's starlets. Keith Ayoob, a registered dietitian and director of the nutrition clinic at the Albert Einstein College of Medicine in New York, warns, "Do not try this at home!"
"They do this for a role," Ayoob told ABCNews.com, "and it's often not the best thing for their metabolisms and bodies. But if they are getting $10 million for the role, I'm not sure the producer cares if their long-term health is at stake."
Here's the skinny on the latest actresses taking their bodies to extremes.
A year before shooting "Swan," 29-year-old Portman went through extreme ballet and cross-training, shedding 20 pounds from her 5-foot-3-inch frame.
"I think it was just the physicality of it all that was the most extreme," she told UsMagazine.com. "I mean, I had never gotten that much training -- to be doing five-to-eight hours a day of [it] was really a challenge."
As soon as filming wrapped, Portman, who danced as a kid, was happy to leave the ballet world behind. She told "Access Hollywood" that she immediately ceased working out and began carbo-loading to bulk up again.
"I was like, pasta, pasta, pasta! No working out," Portman said. "It was pretty immediate. I was ready to leave the ballet life.
"I was like, 'Please don't let there be reshoots ... because I don't think I could get back into the costumes!'"
However, with Oscar buzz growing for "Black Swan," which opens today, Portman has said she would do such extreme training again.
"It's always one of those things where, when you put in a lot, you get out a lot," she told Us.
Maybe a gold statue?
Portman's "Swan" co-star Kunis also dropped 20 pounds to play a ballerina.
For three months, the "That '70s Show" star trained, through ballet, cardio exercises and Pilates four-to-five hours a day, seven days a week.
In the end, she became frightened by her own reflection. "I could see why this industry is so f**ked up, because at 95 pounds, I would literally look at myself in the mirror, and I was like, 'Oh my God!'" the five-foot-three-inch actress told E! Online. "I had no shape, no boobs, no ass. ... All you saw was bone. I was like, 'This looks gross.'"