TORONTO, Ont. - Carey Mulligan couldn't stand the thought of another actress playing the lead role in the big-screen adaptation of the beloved book "Never Let Me Go."

The Oscar-nominated performer says she fell in love with Kazuo Ishiguro's acclaimed novel, about three friends whose lives are torn apart after they leave their idyllic boarding school.

"My agent said to me last year, 'You should only ever take roles that you can't bear the idea of anyone else taking and I can't bear the idea of anyone else playing Kathy," Mulligan said Saturday during a round of interviews at the Toronto International Film Festival.

"I think other people could do it brilliantly and probably better -- but they shouldn't."

The 25-year-old Mulligan plays the restrained but strong-willed Kathy in Mark Romanek's vision of the taut sci-fi novel, set in an alternate world in 1990s England.

Keira Knightley plays her vivacious and manipulative best friend Ruth and Andrew Garfield completes the love triangle as the sensitive Tommy.

Ishiguro's 2005 novel was lauded as one of the best of the decade for its melancholy tale of love, betrayal, sacrifice and mortality. Mulligan was an immediate fan when she read it at age 22.

"I was really, really into the book and I knew they'd make a film. I just hoped that they'd wait. I hoped that it would happen when I was old enough to play Kathy," said Mulligan, who earned an Oscar nomination in 2010 for her portrayal of a schoolgirl who falls for a con man in 2009's "An Education."

"It was a real favourite of mine and I thought if they mess it up it'll be horrible. And they didn't. I felt like the script was perfect and the people who were already involved by the time I came on board were perfect."

Garfield, who also stars in the upcoming Facebook film, "The Social Network," and was recently cast as the next Spider-Man, said he was drawn to the heavy existential questions the story raises.

"There's a sense of anxiety that runs through these kids, especially Tommy, because he's so sensory and feeling and animalistic, that's my perspective of him," said Garfield, who wore a "Back to the Future" watch and gushed over '80s film favourites including "Teen Wolf."

"(He's) very instinctive and intuitive, so he can't intellectualize the situation as well as other people, but he's aware that there's something not quite right."

Romanek said he's often asked why the young characters don't rebel against the circumstances they're forced into. He says he thinks the alternative -- to stay and accept your lot -- is far more interesting. His approach was heavily influenced by Japanese film and art from the 1950s.

"It's a very American idea, I think, to fight and run," said Romanek, who directed "One Hour Photo," and chose a shabby, muted colour palette for his most recent film.

"I'm not an expert on Japanese culture but it's my understanding that it's considered heroic to fulfil one's duty to society. It's a more Japanese conception and the American conception is almost a 180. I live in London now and I'm still a bit shocked by the kind of class system that exists there that doesn't exist in that same way here. It's frowned upon for people to try to rise above their station in life."

"Never Let Me Go" opens in Toronto and Vancouver on Sept. 24 before heading to Calgary and Edmonton on Oct. 8 and Ottawa, Victoria and Winnipeg on Oct. 15.