It started with a single image: A mother and her son sitting on a train.

Soon, that engine of parent and child pulled the rest of the story from writer-director Noah Baumbach's imagination. The resulting movie, "Margot at the Wedding," opens Friday in Pittsburgh.

It stars Nicole Kidman as Margot, a writer who decides to go to her sister's wedding and brings one of her two sons along. Margot is a writer, bright and successful but opinionated and honest to the point of insensitivity or tactlessness.

She tells her sister, played by Jennifer Jason Leigh (who happens to be Baumbach's wife), "Pauline, what are you doing getting married to him? He's like guys we rejected when we were 16."

Not exactly what a bride-to-be wants to hear.

Baumbach, an Oscar nominee for his original screenplay for "The Squid and the Whale," didn't know where his flash of a mother and child on a train would lead.

"I very quickly started to think about the notion you have of a family unit -- in this case, parents and two boys -- and when a parent and one child take a trip away from the rest of the family, how that dynamic changes in the world at large," he told a handful of reporters during the Toronto International Film Festival.

"I think everybody falls into their roles but when you go outside, just two of you and you go into strange territory, often those dynamics can shift." In this case, they're accelerated by the son's puberty and Margot's indecision about her own marriage.

The idea of the wedding emerged later.

"I knew fairly quickly they were visiting her sister. The wedding came ... a couple of months later, when I was deeper into it," as the characters became more defined.

Baumbach, who drew upon his emotions when his parents split for "The Squid and the Whale," again taps into his familiarity with writers and teachers for "Margot."

"I am interested in people who are articulate and can express themselves in sophisticated ways and know a lot about themselves, but because there's always stuff you don't know, too, it's interesting to me how a seeming rationality and sophistication can hide chaos."

That would be the case with Margot, who is played by the director's first choice of Kidman, thanks to his cachet from "Squid."

"It's safe to say before 'Squid and the Whale,' I might have been arrested trying to have a coffee" with the Oscar winner, he joked. Getting Kidman to co-star in his movie, though, was the "easiest experience I've ever had in the movie business, oddly."

He had coffee with her and handed her the script.

"We're both shy people, so it was a kind of quiet, halting conversation. She took the script home, and I got a call the next morning saying, 'I'd like to do it.' And this is coming off a five-year process of trying to get 'Squid and the Whale' made, where I couldn't get anybody to finance it or to be in it."

It was a very nice change.

"I prefer it this way," Baumbach said, sitting at the head of a polished conference-room table at the Park Hyatt Hotel. He had 42 days to shoot "Margot," rather than the 23 for "Squid," which made for a kamikaze mission further pressured by a ceiling on the modest budget.

Forty-two days "is still not a lot by Hollywood standards but I like it that way. I like having a kind of energy, the actors like it, they get to work more," and wait around less.

If "Squid" was about the coziness of a family being disrupted, "Margot" is about being unable to find your bearings. "It's about being thrown into an experience that's fraught with all these expectations and anxieties and relationships that keep shifting and turning, and the past and the present," he said.

And, just because someone has children doesn't mean she can't go through her own crises.

"I think Margot's having a hard time being a mother in a lot of ways, she's also feeling ambivalent now about the marriage. She has these expectations about her sister, who she's very critical of, but she also considers her closest friend in the world."

The crises were familiar to Leigh, who had read various drafts of the script while her husband worked on it. While one interviewer said she could never collaborate with her spouse, Leigh was thrilled.

"An idealized version of a marriage is when the couple works together," said Leigh, who started acting as a teenager and graduated to such movies as "Fast Times at Ridgemont High," "Last Exit to Brooklyn," "Mrs. Parker and the Vicious Circle," "Dolores Claiborne" and "Georgia."

"I was excited to work with him. I kind of wanted to show off for him a little bit, you know? 'Cause he knows me as his girlfriend or his wife, all these different ways, but he's never seen me on set and I work really hard," Leigh said in a separate session with reporters.

"I'm really good at what I do, and I'm very easy to work with, so I was excited for him to see that. I couldn't wait for him to see what a good girl I was," she said with a laugh.

After all, the tabs and trades are full of stories about high-maintenance stars but rarely about the pros. If the 45-year-old is well-behaved on set, it could be because she was born to the business as the daughter of actress-screenwriter Barbara Turner and late actor Vic Morrow.

She said it was effortless to play Kidman's sister (and the two look like they could be related, although the Aussie is much taller). "She's just such a brilliant actress, and she's so strong and she understood the character so well. I'd been living with these characters for a very long time," Leigh said.

The movie's topic opened the door for others to share their sibling stories.

"Women, especially, can enjoy it in a more painful, primitive way and maybe enjoy isn't the right word," Leigh said. "Men also recognize their sibling dynamics, too. I think it's pretty universal stuff."

While Margot may plant doubts about the groom-to-be, played by Jack Black (another first choice for Baumbach), Leigh considers them a good couple.

"I think they're very well-suited. I think he makes her laugh. I think he's very easygoing. He's very loving. He's very unjudgmental. He's slightly depressed, he doesn't know what he's doing with his life but he's very accepting and loving with Pauline, and you get the feeling with Pauline that she didn't have a lot of that growing up."

But she sees him through new eyes and suddenly she doesn't trust the relationship. "It's a very slow but potent poisoning that goes on."

Margot is using Pauline's wedding to escape from her life while she tries to figure out what she wants. "She can't help but destroy everything around her because she's filled with so much self-loathing," Leigh says.

The movie taps into the well of emotions -- love, jealousy, sharing of intimacies -- surrounding siblings and "this movie is really about how hard it is to break free of the family you came into the world with and choose another."