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The erotically charged and intensely dark Black Swan is sure to ruffle feathers amongst Natalie Portman fans.

The role, which sees the star play ambitious ballerina Nina Sayers, marks a complete change for the former Star Wars actress.

The 29-year-old performed most of the dancing in the movie herself, having trained for a year before filming started.

She also lost 20lb (9kg) from her already slender frame.

"I would train for about five hours a day with a ballet teacher," she says. "I would swim a mile a day and then tone for two hours.

"I don't think I expected how hard it was going to be."

Once shooting started, the pace got more intense. Despite several injuries, including a dislocated rib, there was no time for Portman to recover.

"It's good to understand what real ballet dancers have to go through," she explains. "They are constantly dancing through very difficult injuries.

"They'll dance beautifully with a sprained ankle or some really extreme injury, and then limp off into a bucket of ice. It's pretty shocking."

The actress - currently expecting a child with her fiance Benjamin Millepied, a choreographer she met while making the film - says she learned a lot from her character.

"I learned how much I could do. I think of myself as someone who seeks pleasure and who doesn't like pain.

"But to actually put myself through pain for that long was a scary thing to discover. It's good to know I could focus in that way for a role."

Directed by Darren Aronofsky, the psychological thriller has drawn comparisons with his previous movie The Wrestler.

Both tell of obsessive athletes who strive for perfection to the detriment of their physical health and mental well-being.

Mickey Rourke won numerous awards for his role in The Wrestler. And Portman picked up the best actress award at both the Baftas and the Oscars.

"It's exciting to be in a film that people like in a year of good films," the actress says modestly.

Indeed, the former child star says the most rewarding part of being in a critically acclaimed movie is the audience's reaction.

"It's really exciting to hear people debating their different takes on what the movie is about.

"Just to see people engaging so passionately is your greatest dream when you're making a movie."

Portman was first approached by Aronofsky - whom she calls "phenomenal and wonderful" - to make the film eight years ago.

Yet work on the film was delayed by the difficulty he faced finding people in the ballet world who were willing to talk to him.

With most of the characters portrayed as extremely competitive and sometimes brutal, Black Swan does not show the art of ballet in a particularly good light.

The movie has been criticised by some of its practitioners, who have called the film's portrayal of dancers cliched.

"Clearly this depicts one particular dancer's story in one particular fictional company," Portman says in the film's defence.

"It's not meant to be taken as truth. But there's a lot of ballet dancers who have verified some of the details of the film."

The time ballet dancers have in the limelight is quite limited - a fact highlighted in the film by Winona Ryder's character, Beth Macintyre.

The 39-year-old plays a former ballet dancer forced out of Portman's company because she is considered too old, who then embarks on a downward spiral of depression and unhappiness.

"She's an icon, and it was really exciting to get to work with her," Portman says of the Beetlejuice star.

"She was kind, professional and patient. She was completely humble and was able to turn on extreme emotion really quickly."

Having been in the industry herself since starring in 1994 movie Leon, Portman concedes the acting industry has "similar pressures".

Yet she points out that growing older is not quite as limiting in the worlds of film and theatre.

"For actresses you can change the kind of roles you go for, whereas for dancers your career is just over when you get to a certain point.

"But ballet is truly an art of passion. No one's becoming rich and famous from being a ballet dancer any more.

"There's something incredibly beautiful about that."

Black Swan opens in the UK on 21 January.