TORONTO -- Like a cat curled up in a pool of sunlight on a wintry day, Christopher Hampton had chosen a chair near an open hotel window, with rays, a warm breeze and noise from the street below streaming in.

Summer in London had been lousy, the writer said, and he was taking advantage of the late-season surge in temperatures at the Toronto International Film Festival. Looking professorial with his long graying hair falling to his shoulders, Hampton was in Toronto as part of the tony team for "Atonement."

Christopher Hampton

He adapted the Ian McEwan novel, and the version you see on the screen, starring James McAvoy and Keira Knightley, is not the first or second or even third he wrote. Nor was Joe Wright the first director on the project.

Hampton initially worked with old friend Richard Eyre on three drafts of the screenplay, with McEwan giving support and counsel. "There was a fork in the road, and Richard chose to do 'Notes on a Scandal,' so suddenly I was told it was going to be this young chap, Joe Wright."

Wright, now 35 but still young compared to many directors, said the script was fine but wanted to start from scratch.

After an "Oh, God" moment, it proved to be an interesting experience, Hampton said, because he had to reconceive the story, a lush tale of love, lies, war and atonement. Hampton, by the way, writes in long hand with pen and paper (someone else types his work) because he finds it's "calmer."

The original draft was longer, more literary, with a voiceover, a framing device featuring Vanessa Redgrave who plays a character in her later years, and simultaneous treatment of World War II scenes on the beach at Dunkirk, France, and in a British hospital. The action cut back and forth.

"Joe said, 'Do you mind trying it like it is in the book, in blocks?' which I did, which seemed to be very successful," Hampton said. Even more crucial, perhaps, may have been Wright's decision to have two actresses play Briony Tallis as a 13-year-old and as an 18-year-old, rather than cheating with a 15-year-old as both.

Briony is a precocious girl who sets a tragic string of events into motion. She is played by Saoirse Ronan as the younger girl and Wright said, "She's a remarkable little actress, isn't she?"

Asked about the riskiness of hanging the early movie on one so young, he said, "You know what? I think it's kind of easier to find a great child actor because they're so un-self-conscious in front of the camera if they've got the right temperament.

"Sometimes they last, and sometimes they don't. You look at Christian Bale in 'Empire of the Sun' and you can see that he's a really good actor and he still is, and I hope Saoirse has a really good career."

It's been two decades since "Empire" was released and almost 19 years since Hampton became an Oscar winner for his adapted screenplay of "Dangerous Liaisons" starring Glenn Close, Michelle Pfeiffer and John Malkovich. He had reimagined his own stage play, which had its roots in Choderlos de Laclos' novel, "Les Liaisons Dangereuses."

Born in the Azores in 1946, Hampton lived in Aden, Egypt and Zanzibar as a child and studied German and French as a teenager.

While at Oxford, he became the youngest writer to have a play staged in the West End, and his promise was born out with the Oscar plus a pair of Tonys for "Sunset Boulevard" and Tony nominations for "Les Liaisons Dangereuses" and "The Philanthropist."

"Atonement," which opened Friday in Pittsburgh at the Manor and AMC-Loews at the Waterfront and will go wider soon, was 10 or 15 minutes longer until the last round of editing. Some of the excised scenes featured Redgrave but the filmmakers decided they needed to get to the end as fast as possible.

Hampton is a big believer in listening to an audience's reaction to a movie, more than reading their reactions scrawled on comment sheets. "They don't tell you very interesting things on those bits of paper, but they tell you things by the way they breathe."

Or don't.