To say that

Natalie Portman's

character in

Black Swan

- a ballerina wound tighter than the bun on her head - undergoes a transformation is to proffer a mammoth understatement.

But for audiences who have grown accustomed to the actress' mostly tempered, tamped-down, sensitive screen work, her performance in Darren Aronofsky's dance-world psychodrama will come as a revelation. As Nina Sayers, a twentysomething New Yorker tapped to star in a production of Swan Lake, taking the lead as the innocent white swan and her dark-souled doppelganger, Portman goes places nobody's seen her go before.

Unless, perhaps, she's been in therapy.

"First of all, there are not many roles like this for women," says Aronofsky, acknowledging that his leading lady is doing something extraordinary in Black Swan. "So Natalie hasn't had many opportunities to do something like this. . . . But she really went for it. And really had no problem going over-the-top, or going more subtle. She just really had fun with it."

Although "fun" isn't exactly a word one would associate with the grueling physical regimen Portman endured - hours of dance training with a choreographer, rigorous daily workouts, a complete change of diet, every day for months before filming began. And then all the way through production, as well.

Black Swan opened at the Ritz East, the AMC Neshaminy, United Artists King of Prussia and Rave Motion Pictures at the Ritz Center/NJ on Friday.

"We talked about the project 10 years ago for the first time," says Aronofsky, the man who brought Mickey Rourke back from the dead in The Wrestler. "And then when I gave her the script a year out from shooting, she started to train, and I think that that type of physical work combined with being that deep into the world of dance . . . gave her a tremendous amount of stuff to draw on. . . .

"But you know, I feel actors really want to act - really going for it is what they love to do. I wouldn't say I'm a pusher, I'd say I'm more of a puller, because when actors work with me they know they're going to get a chance to go for it. . . .

"I'll just show them how much space there is to explore, and they're very willing to step in. . . . I guess I had to push Mickey a little bit, but that's just because he's, you know, lazy. But Natalie was game. . . .

"And it wasn't just the physical work. The emotional stuff, for her, was intense as well."

Like The Wrestler, Black Swan was shot with handheld cameras. In the same way that Aronofsky took the viewer right up to Rourke's Randy "The Ram" Robinson as he was pummeled in the ring - and pummeled in his life - the camera dogs Portman's Nina through her rehearsals, on her subway commutes, and as she deals with a smothering mother (Barbara Hershey), with the company's demanding artistic directior (Vincent Cassel), and with the threat of a new dancer, fresh in from the Coast (Mila Kunis).

"I really wanted the camera to dance with the dancers, with Natalie," he explains. "I wanted to have that fluidity of movement, but I was a little nervous about bringing those kind of documentary/cinema verite approaches to a suspense film - I wasn't sure if it would suck out the suspense. And I couldn't find an example of a film that had done that. So, that was a bit of a risk. . . .

"But we really get in there. And that came from The Wrestler, because wrestling on TV is always shot with these three cameras, and I wanted to do what they did in boxing movies with wrestling. So I got the camera into the ring.

"And then I took the same idea to ballet, because what I noticed when you watch ballet from the audience, it just really looks effortless. But when you get backstage and you get close to these dancers you see the sweat, you see the breath, you see how hard it is, you see the blood, you see the pain. And I just knew I needed to be up there."

Another reason for bringing the camera up close and personal was more technical, more for credibility's sake.

"Natalie, from the waist up, is really, really believable as a ballet dancer, but when you start getting into the 20 years you need to train legs to hyperextend and go on pointe, you run into problems," says the director, who used a pair of professional dancers as his star's double in the full-body performance shots.

"So, I had another motivation to get close to Natalie as well. But also, because she's a damn good actor, and I want to see into her eyes."

Black Swan is a kind of brazen mash-up of movie classics: Michael Powell and Emeric Pressburger's crazy fairy tale The Red Shoes; Roman Polanski's tale of repression and madness, Repulsion; the showbiz paranoia of Joseph L. Mankiewicz's All About Eve.

"Sure, sure," says Aronofsky. "It's interesting, because there are so many influences. And as a filmmaker you're always watching stuff. . . . And then from a technical side, there were the two Frederick Wiseman documentaries on the ballet world, one on the Paris Opera Ballet and the one on the American Ballet Theatre."

Aronofsky also cites the gritty, visceral work of the Belgian filmmaking brothers Jean-Pierre and Luc Dardenne.

"Those guys are all in there, too, so it's hard to say where exactly everything comes from."