WHAT'S NEXT - An active imagination - Saoirse Ronan, 13, is the emotional anchor in 'Atonement' and, soon, 'The Lovely Bones.' It's all a matter of being.

"She was one of those children possessed by a desire to have the world just so," writes Ian McEwan in his novel "Atonement." He is describing 13-year-old Briony Tallis, one of recent literature's most maddening heroines, brainy but impetuous, controlling but immature and blind to the cues of her heart and others'. "Her wish for a harmonious, organized world denied her the reckless possibilities of wrongdoing," McEwan writes -- an ominous foreshadowing of how Briony, in her ferocious need to make sense of a narrative beyond her ken, naively destroys her sister's and her lover's lives. It's the act that dooms the young girl to a life of "Atonement."

The film "Atonement" opens on Briony tapping away on her manual typewriter her first play, "The Trials of Arabella." Snatching the completed project, she determinedly marches through her grand mansion of a home, her thin, tiny body turning corners at precise right angles, her gauzy white child's dress not quite covering the manic certainty with which she holds herself. While the posters of "Atonement" triumph the presence of the grown-up star Keira Knightley, the film wouldn't work without the innocent destructiveness of 13-year-old Saoirse Ronan.

"This is what I try to do with everything -- is just be the person," says Ronan, at breakfast with her parents in a Los Angeles hotel. "Be the person I'm playing. That's what acting is. You're pretending to be someone else."

Ronan flew in the previous night from Pennsylvania to attend the "Atonement" premiere, and she's leaving in a few hours. In person, she has none of Briony's strait-laced quality. Dressed in a striped T-shirt and high-waisted jeans, she looks as limber and slouchy as any teenager, with a clear, open face and shiny brown hair that tumbles down to her shoulders. She has a casually cheery manner far from the automaton, preternaturally grown-up quality of some child actors. More surprisingly, she speaks in a thick Irish accent.

DAD AS ACTING PARTNER

Ronan might be an acting savant. In a time when there are more than a few talented 13-year-olds around (such as Abigail Breslin and Dakota Fanning), Ronan has managed to scoop up a raft of the most desired children's parts. In the coming year, she stars opposite Bill Murray in "City of Ember," director Gil Kenan's live-action fantasy follow-up to "Monster House." She plays Catherine Zeta-Jones' daughter and con partner in "Death Defying Acts" and, most notably, inhabits the role of 14-year-old Susie Salmon in Peter Jackson's upcoming screen version of "The Lovely Bones."