But, together with a cast that also included Vanessa Redgrave and 13-year-old newcomer Saoirse Ronan, they brought the pages of the best-selling novel to life in the film that opened the Venice Film Festival on Wednesday.
Wright's "Atonement" opens in the English countryside during a steamy and idle 1935 summer that allows a privileged adolescent girl's fantasies to get the best of her after she witnesses events beyond her experience. It culminates against the backdrop of World War II.
The film gets its tension from a false accusation by the fanciful Briony Tallis (Ronan) against her elder sister's lover, altering the course of their lives.
Knightley plays Cecilia Tallis, whose budding romance with the son of her family's housekeeper, Robbie Turner (McAvoy), is upended by Briony's accusations.
Wright acknowledged the awesome task of bringing to screen a novel loved by so many.
"I think it is important to be true to your own experience of the novel. If you are truthful, the audience will feel it too," Wright said. "You just do what you think is right."
Wright confessed that he hadn't read the novel before getting involved in the project -- and conceded that reports on Internet Movie Database that he was dyslexic and didn't have much of a head for school were correct.
"I like the fact that IMDb has described me as stupid," Wright joked. "Unfortunately, it is true on all counts. Guilty, guilty, guilty."
He was in good company. Knightley said her mother had given her the book but she had never gotten around to reading it. And McAvoy said that after screen-testing with Knightley, he wouldn't have been able to read the book at all if he hadn't gotten the part: "It would have been too painful."
Knightley said she cried when she read the screenplay by Christopher Hampton -- which she took as a sign that she should do the film.
"When I first read the script I just completely fell in love with Cecilia, totally and utterly, partly maybe it was because of the romance, and I like a bit of romance," said Knightley, whose roles have run the gamut from the romantic to the swashbuckling in the "Pirates of the Caribbean" trilogy, and to the action-thriller in John Maybury's "The Jacket."
Wright and Knightley collaborated previously in Wright's directorial debut "Pride & Prejudice" and more recently on Knightley's commercial for Chanel as the new face of Coco Mademoiselle.
Knightley addressed suggestions that her slender figure had been enhanced for the spot, saying: "I haven't had curves added to my body for a perfume, I don't think, unless you've seen something that I haven't."
She has previously discussed retouches on her figure on publicity for films -- and told reporters in Venice she had nothing to atone for.
"I think films particularly deal with fantasy. I think that it's very important and actually what this film shows is the danger when the line between fiction and fact gets blurred.
"I think that people have to be very honest about fiction, and magazine pictures for example are fiction. One person has done the makeup, the hair, it's somebody else's clothes, it's choreographed by somebody else."
"Atonement" is set to open in the United States in December.
By COLLEEN BARRY