TORONTO–Some relationships have weird beginnings. Noah Baumbach and Jennifer Jason Leigh's relationship is a case in point. Baumbach was 13 when he watched the coming-of-age comedy Fast Times at Ridgemont High. He has since told reporters that he had always thought it was a drama because the only character he was paying attention to was Leigh's, a 15-year-old with an unwanted pregnancy.

Leigh was 20 when she made Fast Times, but Baumbach grew up with a crush. He met her a few years ago through his friend Eric Stoltz, who had dated Leigh. The two married in 2005. She helped him through the making of his breakthrough film, The Squid and the Whale, and soon collaborated with him on Margot at the Wedding.

The movie, which opens Friday (December 7), stars Nicole Kidman as Margot and Leigh as Pauline, two sisters who try to put aside their differences for Pauline's wedding. In a hotel room during the Toronto International Film Festival, Leigh said that when Baumbach showed her his drafts, neither talked about whether she would have a role in the film.

"I knew at a certain point that he was thinking of me for Pauline, but he doesn't have actors in mind when he writes," she said. "Since it was mostly about what these two women mean to each other, I could say to him, 'Let's talk about how this would work.' I had no attachment to a role, so I was able to read it objectively and to be helpful to my partner. The actual acting of the roles was never part of our discussions, but when he told me he wanted me to be Pauline, I was happy."

Neither Pauline nor Margot are particularly likable. The married Margot is having an affair with a man who lives near her sister, and Margot is using the wedding as an excuse to see him. And Pauline begins to question her own relationship with her fiancé (Jack Black) when Margot decides she doesn't like him. Leigh says she spent so much time reading the script while Baumbach was developing it that she just accepted the perils of playing a somewhat unsympathetic character.

It's not as though Leigh has spent her career playing warm and tender characters. Her previous roles have included a psychotic killer (Single White Female) and an assortment of drug addicts, alcoholics, and prostitutes.

"I do like Pauline…but it was something that I had read so many drafts of and we had talked about it so many times that it made it an effortless transition for me," Leigh explained. "But sometimes it is fun to play characters that you don't necessarily like or empathize with…because you have to figure out a way in, and that becomes its own puzzle and challenge.

"What you want to do is to make them recognizable not only to yourself but to someone viewing the film," she continued. "It can be a nasty bit of fun. I have played lots of characters I wouldn't want to be in a room with. We all want to be liked so badly in life, and some of these characters just don't give a shit.…We wouldn't want to be them because they are kind of horrible, but there is a little part of us that wants that."

That approach to acting has helped Leigh, the daughter of an actor (Vic Morrow) and a writer (Barbara Turner). Fast Times was her breakthrough, but she was on sets as a baby, and she said she took on acting as a career as a way of attaining self-confidence.

"Acting is a way of overcoming my shyness, and it is also a way for me to communicate," Leigh said. "I think a lot of actors are shy but there is a part of them that is dying to…come out of that and be expressive and all of these things that they don't allow themselves to do in life. They feel it's too dangerous or too unpleasant, but on a set they are safe. I know that I'm safe there."