In Atonement, the powerful new film by Pride and Prejudice director Joe Wright, emerging Irish movie star Saoirse Ronan gives an Oscar worthy performance that may lead to a Best Supporting Actress nomination.

CAHIR O'DOHERTY talks about her to fellow actors Keira Knightley, James McAvoy and Oscar winning screenwriter Christopher Hampton and about bringing this epic tale to the screen.

ATONEMENT, the film most likely to blaze an unstoppable trail to the 2008 Oscars, opens on a blisteringly hot day in England in the summer of 1935, when 13-year-old Briony Tallis (played to perfection by Irish newcomer Saoirse Ronan, also 13) witnesses an intimate moment between her sister Cecelia (Keira Knightley) and her lover (James McAvoy).

Misinterpreting what she sees, young Briony later falsely accuses the man of rape, resulting in his imprisonment and separation from Cecelia. The product of an overheated imagination, Briony's impulsive rush to judgment ruins the lives of both participants and sets a tragedy in motion that haunts her for the rest of her life.

The epic scale of the new film - which has been compared by some critics to Gone With The Wind for its decades long storyline and its raw emotional impact - is simply dazzling. For actress Keira Knightley, making the film was like taking a master class in the acting styles of Hollywood's legendary leading ladies.

To delve into the atmosphere and attitudes of the times, Knightley spent weeks studying the films of Greta Garbo, Marlene Dietrich and Katherine Hepburn. But it was a much less known British actress of the 1940s who was her biggest inspiration.

"For me the main acting inspiration for this film was Celia Johnson from the 1940's film Brief Encounter. I watched that film on a loop for two months and I'd actually be happy to watch it on a loop forever," she said.

"As a cast we watched a lot of David Lean and Noel Coward films, and a lot of news footage from that time as well."

For Knightley and the entire cast of Atonement, the biggest struggle was trying to get a handle on the attitudes and accents of that far off era. The upper crust accent of 1939, Knightley discovered, was such a specific thing and completely lost to her own generation. It's what she calls that British 1940's thing, the height of the Stiff Upper Lip era.

Luckily, the entire cast volunteered to watch the old films together so that each of them was on the same page. It wouldn't have worked, says Knightley, if one person in the cast hadn't done it.

Asked about the sensational chemistry between her and leading man McAvoy, she laughs and looks away for a moment. It's not something she feels comfortable talking about, at least not at first.

"I should imagine that James would say it's our job. I really got on with him because he's an amazing actor and working with him was really exciting," she says.

"I think as far as romantic chemistry goes you can have the best actors in the world - and they can even be in love with each other - but for some reason you won't have chemistry on the screen. I don't know that anyone knows what makes that final bit of chemistry work. Obviously we got on and the script is fantastic, but actually I don't know."

In Atonement Knightley plays an upper class woman who could be described as aloof and at times even forbidding, especially at the beginning of the film. For such an intuitive actress was she a bit concerned she might lose the audiences sympathy at times?

"I never saw Cecilia as unsympathetic," says Knightley. "Very often in films you have characters that are black or white, and what's fascinating about her is that she's a very good person but she's behaving like a bitch.

"And I think we all do at times, and you rarely get to see that in film. I found her totally fascinating."

For Knightley, the scene that encapsulates the whole atmosphere of the film is the one that occurs in a packed but genteel British teashop, where the two lovers are finally reunited for the first time in six years. All they want to do is pour out their feelings for each other in

a huge melodramatic moment, but they're surrounded by other patrons and they can't. It's a masterful scene that's all about what's not said, as opposed to what is.

Asked about the accomplished performance given by Saoirse Ronan in the film, Knightley cannot disguise her admiration. "Saoirse is amazing," says Knightley, laughing. "She was 12 when she made Atonement and she has this thick Irish accent.

"But she arrived on the set with this pitch perfect 1930s British accent. I think what's incredible about Saoirse is that her talent is not taught. Where did that talent come from?

"I'm often asked what advice I'd give her. I'd never dream of giving Saoirse Ronan advice. I take advice from her."

Best known for playing historical roles, Knightley insists that it's not because she prefers to do things that are historical, it's simply because those are the scripts that have interested her most.

Recently she's played the iconic role of Elizabeth Bennett in Pride and Prejudice, and now she's starring in another period piece in Atonement. The question is how does she prepare herself for roles that are that ambitious?

"I don't know! I've been very lucky, since those were the ones I got," says Knightley. "As a woman what I'm interested in is seeing good female roles on screen. I'm not interested in token roles."

Asked about the response to her performance that really matters, the one given by writer Ian McEwan, Knightley has a surprising answer. "He came up to me at the London premiere," says Knightley. "He said the film worked well but I played Cecelia so differently than how he had written it in the book.

"And I thought to myself, No I didn't! I played it exactly as you wrote it! And I think that's what wonderful about the film, that people have so many ways of seeing the characters."

McAvoy, 28, who has made the transition to leading man with remarkable ease in 2007, was recently seen in another period film, Becoming Jane.

In Atonement, however, romance eventually takes a back seat to the graphically depicted horrors of World War II. One scene in particular - the full-scale evacuation of Dunkirk - provides a terrifying reenactment of that dreadful event.

Exhausted and stranded British forces, including McAvoy, are seen scattered along the beach enduring the carpet bombs of German fighter planes and the advancing German army. It looks like hell on earth as the British soldiers wait in hope of a rescue fleet. Shooting the scene brought McAvoy right into the heart of the film and the war at its center.

"To find a film that was so sweeping and romantic and yet so intelligent was very nice to me. It's a classic story but it's told in a very contemporary, modern way," McAvoy says.

"I liked that strange structure. People are going to think they watching a Merchant Ivory film for the first 20 minutes and then it takes off in a completely unexpected direction."

McAvoy comes from a Scottish working class background, and he found it easy to step into the shoes of his character. Playing Robbie Turner, the son of a housekeeper who falls for an heiress, he has the skill and the experience to bring his character convincingly to life.

Next year will be a packed year professionally for the young actor, who will be seen alongside Reese Witherspoon and Christina Ricci in Penelope, and then with Angelina Jolie and Morgan Freeman in Wanted.

Atonement screenwriter Christopher Hampton (who won an Oscar for Dangerous Liaisons) is full of praise for the work of the young Irish actress at the center of the film.

"To have Saoirse Ronan - never mind the fact that's she's absolutely brilliant in the role - in the first half of the film really explains the story in a way that wouldn't be explained by an older actress pretending to be a 12 year old. She did an outstanding job and the finished film moves like an express train. I am incredibly proud of what our cast and crew attained."