"He'd say, 'Oh, Mila is doing really well on her stuff. She's so much better than you'," the 29-year-old actress said, referring to her co-star, Mila Kunis.
"Darren would tell us things about each other to try to make us jealous. I think he was trying to create a rivalry in real life between us."
That Aronofsky may have tried to stoke competition between his lead actresses is understandable - envy is at the core of Black Swan, a mystical ballet thriller about an uptight dancer named Nina (Portman) who becomes obsessed by the threat posed by blithe new company member Lily (Kunis).
The young women are vying for the lead role in Swan Lake, and while Nina can perfectly encapsulate the virtue of the white swan, she struggles to convey the sinister, sexual nature intrinsic to the black swan that seems to come naturally to Lily.
"We were really great friends before production. We are really great friends now. And during production, we were working together," Kunis (27) explained.
Aronofsky denies fuelling a rivalry but says he distanced the actresses so that they could not discuss their acting approaches.
"I knew it might be really hard to keep them apart because they're friends, but I just didn't want them to know each other's motives," he said.
"I didn't want them to compare notes. I wanted them to come from different places."
Aronofsky, the film-maker behind Requiem for a Dream and the 2008 Mickey Rourke comeback picture The Wrestler, had his own foes to overcome to get Black Swan made. He decided nearly a decade ago that he wanted to do a film about the ballet world, but several ideas and scripts bogged down in development and the project lost and regained financing numerous times.
The director first met Portman when she was 20. She had taken ballet classes as a girl and had always imagined she'd be a dancer if she weren't an actress, so she was struck by Aronofsky's idea. As Black Swan remained in limbo, she acted in other movies, such as Zach Braff's Garden State and two Star Wars films.
While Portman was long slated for the movie, Kunis (best known for her role in the long-running sitcom That '70s Show) was brought in only months before production began. Portman, who knew that Kunis had dance experience, recommended her friend to Aronofsky. He had seen Kunis in Forgetting Sarah Marshall, and the two met via iChat. A few online video conversations later, he had hired her. But from that point forward, the actresses had little interaction.
Their lack of conversation is particularly interesting, considering they largely play the same character for much of the film. As Nina becomes paranoid about Lily stealing her role, she begins to have delusions - sometimes she believes she's looking at Lily, only to realise she's visualising a darker and more liberated version of herself. The fluidity of that relationship culminates in a heated sex scene between the young women, which is teased in the movie's trailer and has for months been the subject of media fascination.
It was one of the few scenes the actresses shot together, and Portman described it as "super awkward".
"I remember the first time we did it, we were both sort of embarrassed and not going for it," she said, sipping vegetable broth at the Polo Lounge in Beverly Hills. "And Darren was like, `Listen. If you go for it, you're not going to have to do it again. If you get all embarrassed and hesitate, you're gonna have to do it 400 times'."
Kunis said Aronofsky promised the actresses the film wouldn't leave his hands until it was released so that the scene wouldn't go viral on the internet. Crew members had their cellphones confiscated so that they couldn't take any photos of the steamy segment.
Meanwhile, Kunis says she is irritated that she has received so many questions about the racy moment.
"How people walk away from a movie like this and the first question out of their mouth is `Is it uncomfortable making out with your friend Natalie?' - it's unfortunate," she said in a separate interview a few days later. Reporters have also been asking both Kunis and Portman about their trysts in their next films, which are strikingly similar, centring on women in casual relationships. (Kunis is starring in Friends With Benefits, while Portman is in No Strings Attached.)
Both women would prefer the interest be focused elsewhere - such as on the rigorous physical preparation they put into transforming into near-professional ballerinas. Even as the film struggled to get off the ground because of financial issues, Portman began training five to eight hours a day with a ballet instructor. She spent time in barre class, swam a mile each day, did toning and muscle-strengthening exercises and sharply reduced her calorie intake.
"Darren claims he never said this, but he definitely was like, 'How thin do you think you can get without being sick?'." Portman said. She apparently took that order so seriously that the director later began to fret over her shrinking frame.
"At a certain point in the middle of the ballet stuff, I thought she was getting way too skinny and I started to make her eat. It started to get scary, and she was starting to look too thin," Aronofsky admits. "But when you work in the world of ballet, these women are so tiny. I just didn't want her to get hurt, so we surrounded her with the right health people."
Both Portman and Kunis, already short and slight, lost 9kg before production even began.
"I looked like Gollum," Kunis joked, referring to the emaciated, bug-eyed creature from The Lord of the Rings. She said she got down to 44.5kg.
"I did not veer off the diet. I got one day off, on my birthday, and I did have a root beer float.
My ballet instructor was like, `Here's your present!"'
The women each suffered injuries during production - Portman dislocated a rib and Kunis dislocated her shoulder and tore two ligaments - but continued dancing despite them.
"She was really living the life of a ballerina in many ways because of the way she was training and that sacrifice," said Portman's dance teacher, Mary Helen Bowers.
"We would meet many days at 5 in the morning, she would go and work a 12-hour day on set and then meet me at the ballet studio or gym afterwards. She wasn't going out with the cast and having beers. It was a different kind of work experience.And I think that's really the life of the dancer. It's so hard on you physically that you just can't really live a normal life."
