THE PERFORMANCE

“I love characters who are essentially flawed. And when you meet Cecilia . . . she knows she’s being horrible, but she just can’t stop herself,” said Keira Knightley of her role as upper-middle-class English beauty Cecilia Tallis in Oscar hopeful “Atonement.”

Knightley’s Cecilia -- whose sexual liaison with the housekeeper’s son Robbie (James McAvoy) precipitates a catastrophic chain of events -- echoes the cut-glass accent and acting of Celia Johnson in “Brief Encounter,” an emotionally restrained style of performance that serves only to heighten the melodrama.

“You’re looking at the 1930s, 1940s, the peak of that British stiff upper lip, don’t talk about anything,” Knightley said. “So you’ve got this brittle being who’s on edge, a woman who’s a bit like a pressure cooker about to explode. The fact she’s redeemed by love, by the sacrifice she’s made, was a really interesting arc to the character.”

Although he directed Knightley to a best actress Oscar nomination for “Pride & Prejudice,” director Joe Wright originally had in mind another role for her in “Atonement” -- that of Cecilia’s younger sister Briony, age 18, who would eventually be played by Romola Garai. “After making ‘Pride,’ ” Wright said in a telephone interview, “I thought of her as this 18-, 19-year-old girl.” When Wright saw the actress at the “Pride & Prejudice” premiere in Toronto, he realized his error. “Keira turned up wearing this Roland Mouret dress, and she’d changed into a proper woman. I realized she was right for Cecilia.”

“It’s amazing how somebody can go, ‘Ah tight dress, she must be a woman,’ ” Knightley said, laughing.

“I’d been playing teenagers, that girl on the cusp of, but not quite there,” Knightley continued. “I never liked being a teenager, I always wanted to be grown-up; that’s why even the roles I’ve played that are teenagers have always been slightly uncomfortable in who they are. I suppose I got very frightened I’d be stuck, at 30, still playing teenagers.”

Given the early response to her work in “Atonement,” Knightley needn’t be concerned about future access to meaty roles. To get to the core of Cecilia, Knightley dived back into the rich source material, Ian McEwan’s novel, finding a psychological and physical underpinning for her character’s initially hostile behavior toward McAvoy’s Robbie.

“The beginning of the book, where it describes Cecilia, it’s all about how much she needs a cigarette,” Knightley said. “The whole vibe was a nicotine addict needing a cigarette and not being able to find one.

“The most helpful part was the description of her bedroom,” she adds. “Books that she had half-read, ashtrays over-spilling that she never empties, the cup of cold tea with the lipstick stains on it. With me it’s always about finding the images that make the behavior make sense.”

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Where you’ve seen her

Best known for her role as Elizabeth Swann in the blockbuster “Pirates of the Caribbean” trilogy, 22-year-old Keira Knightley has been working steadily since she was a child. Soccer romance “Bend It Like Beckham” launched her career stateside, leading to turns in “Pirates” and the starring role in Tony Scott’s “Domino.” But it was her Academy Award-nominated lead performance in “Pride & Prejudice” that made people take note and see her as more than a pretty face.