NEW YORK •On a recent afternoon under a brooding artificial sky, actress Keira Knightley was once again sitting in a rocking boat on a churning body of water.

Men were fighting over her, as usual, and one of them was not going to make it to shore. But Knightley was yawning anyway.

The dinghy in question was safe, mind you, surrounded by the walls of Studio 54, where the actress is starring in the title role of the Roundabout Theater Company production of Therese Raquin, Helen Edmundson's stage adaptation of Emile Zola's classic 19thcentury tale of love, guilt and the critical importance of swimming lessons.

While the sight of a movie star wedged in a rowboat - it bobs in a shallow pool devised by set designer Beowulf Borritt - was odd, perhaps the more telling detail was that yawn, which during this particular technical rehearsal was likely born more from exhaustion than ennui.

Knightley, after all, is preparing to make her Broadway debut on Oct 29, just five months after another first: the birth of a little girl named Edie, whose toys, blankets and nappies are a common sight in her dressing room at the theatre.

She has discovered that motherhood, like theatre, can be dirty work.

"We're doing note sessions and I'm literally cleaning the snot off my daughter's face," she said.

"And my husband's trying to change her diaper as she rolls along the floor," she said, referring to English musician James Righton.

She seems equally enthusiastic and intimidated about both debuts, speaking in similar terms about the role of Therese - "It's as dark as they get, you know, sort of like jumping off a cliff into hell" - and the intensity of a new mother's love.

"You give birth to something and you go, automatically, 'I would die for it', and you don't feel like that for any other thing," she said, adding: "This little thing that's screaming at me and there's absolutely no question. And that's dark and it's deep and it's very, very, very strange."

The play rests on the actress' drawing power and Edmundson's reputation as an elite interpreter of highbrow literary fare.

For an actress known for her ability to sparkle and charm in several accents, Therese Raquin is something of a change of pace.

As imagined by Edmundson, Knightley's tragically repressed Therese is onstage for nearly the show's entire 21/2 hours. And for the first 30 minutes or so, she barely speaks, only periodically having a one-sided conversation with... a river.

But Therese's life quickly gets more passionate (thanks to Laurent, a handsome friend of her husband, played by Matt Ryan) and poisonous (Laurent, again).

Depending on one's viewpoint, the character is either a victim of love or just marginally insane, complete with the violent mood swings that either condition implies.

It is a role that in some ways mirrors Knightley's cinematic take on Anna Karenina, another woman whose affair with another man goes off the rails.

"I get very interested in people who are caged in some way," she said, "and I think it's quite true that very often, people who try to break out of their perceived cage do get punished for it, whatever that cage is."

She had been offered the role of Therese Raquin twice in her 20s, which she admitted to turning down because it was something "I don't know how to do".

But last year, when her agent sent her Edmundson's adaptation, she figured it was a sign.

"When it came back to me the third time, I thought, 'Well this is weird'," she recalled. "And I am still frightened of it and I don't know how to do it and there are so many problems with putting it on. But I was sort of up for the challenge."

She agreed to do the role, however, before she knew she was pregnant and her initial instinct upon finding out was to withdraw from the production.

But a coterie of female friends and family argued otherwise.

She said she may well want "a complete piece of frivolity" after tackling Therese.

But regardless of her reception on Broadway, it sounds as though her plunge into theatre - and dark roles - may continue. "I think, actually, that's why I enjoy theatre: It doesn't exist. It's not like film, which is stuck there forever."

She continued: "The show that you see tonight, whether it works or whether it doesn't work, nobody will ever see that again. Tomorrow will be completely different. And I find that incredibly romantic."

NEW YORK TIMES