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In the blue corner! Boasting a string of heavy-hitting stage and screen credits, a deft range and a quirky killer smile, and weighing in at whatever it takes, it’s the 25-year-old London Lightning Bolt, RRRRRRomola Garai!!! And in the red corner? It’s that skinny star with the squeaky voice, Keira Knightley.

See! This is just the sort of thing that drives Romola Garai crazy. “Lazy, salacious, journalistic nonsense” is how she describes the concerted media effort to pitch her, tooth’n’claw, up against her alleged arch-rival and Pirates of the Caribbean pin-up Knightley. The two have apparently been scrapping over choice roles for years.

The epic television adaptation of Dr Zhivago and the Oscar-nominated Pride and Prejudice were just two of the titles that launched Garai and Knightley into a casting head-to-head (and though Garai lost out to Knightley in both, one heavyweight critic was smart enough to note that Garai’s competing epic Daniel Deronda blew Knightley’s Zhivago off the screen). And so now that the two leading ladies, like De Niro and Pacino in Heat, are finally appearing on screen together in a stunningly precise adaptation of Ian McEwan’s Atonement, the temptation to reignite the duelling egos debate is overwhelming.

“But I just don’t think it’s true at all!” says a plaintive Garai, from the corner of her hotel room in Wellington, New Zealand, halfway through an RSC world tour of King Lear and The Seagull. “She’s a massive movie star, and pitting us against each other is demeaning to everyone involved.

We’re more or less the same age – I’m a few years older than her – and we do sometimes go up for the same job. But it doesn’t really extend to anything more exciting than that!”

And yet, if you buy the whole Knightley-Garai smackdown, there’s a certain poetic justice to Garai’s arresting turn in Atonement. The actress, who arrived in the mainstream with an award-winning performance in 2003’s I Capture the Castle, here plays the blue-blood Briony Tallis, the younger, plainer sister to Knightley’s glamorous Cecilia. It’s the eve of the Second World War, and both sisters are in love with the same man, Robbie (James McAvoy). However, the unexpected sight of Robbie and Cecilia in flagrante is enough to send Briony into a traumatised tailspin, and sparks an impulsive act of revenge that will have grave implications for those involved, and will ultimately motivate Briony’s lifelong desire for the atonement of the title. Here, rather than fighting against type, or competing for wattage with Knightley, Garai embraces the strange, impenetrable otherness of Briony, complete with dowdy nurse’s wardrobe and tortured bug-eyed stares.

“Briony’s weird,” she admits. “I wanted her to be weird, and not sexy, though. Because if I’ve learnt one thing, at 25, it’s that what stands in your way a lot as an actress is constant pressure on you to be beautiful – it certainly stands in the way of properly developing a character.”

Now, this is clearly not aimed at Knightley’s babe-a-licious screen persona, but the reality of Atonement is that, though Knightley’s performance is a bravura depiction of tremulous unrequited love (at times veering perilously close to an imitation of Celia Johnson in Brief Encounter), it is ultimately Garai’s turn that fascinates. She is the most modern character in the film, defined by her own voyeuristic desires and a sense of alienation and guilt. And in a remarkably accomplished movie that is epic in scope, intimately dramatic, and bound to dominate the awards season, hers is the star-making turn.

Garai, however, is typically sceptical about the promise of super-stardom. She says that she’s been here before – Garai was deemed a hot prospect after I Capture the Castle, called a “young Cameron Diaz” by Variety, and again lauded after her award-winning turn as a blunt-speaking carer opposite James McAvoy in the 2004 wheelchair drama Inside I’m Dancing. Instead, she says, without false modesty, that she simply wants “to do the jobs that I want to do and let the work speak for itself”. Hence the world tour with Chekhov and Shakespeare at a time when other actresses might be, well, pressing the flesh.

In fact, Garai is refreshingly off-message about the promotional aspects of the movie business. She has a rapier intellect, and can deconstruct a movie’s subtext with alarming insight – her analysis of the sexual dynamics of Atonement would make a Freudian scholar weep with joy. But she refuses to play compliant media ball. Thus, she’s quick to interrogate questions, and even quicker to spot logical flaws (the suggestion that Atonement is a vaguely “independent” movie is immediately shot down with a detailed, and correct, account of the movie’s status, via Working Title Films and parent company Universal, as a mainstream Hollywood effort).

This instinct, engaging and frankly innovative in a world of promotional drones, has occasionally earned Garai the wrath of journalists, who have been affronted by her decision to have a personality. “I don’t read everything that’s written about me,” she says. “But it filters through. And I know that for everybody that thinks I’m talented and promising, there is somebody out there that thinks that I’m s***!”

Born in Hong Kong to a high-profile banker of Jewish-Hungarian descent, Garai was raised in Singapore until the age of 8. She says that these pivotal years abroad as the archetypal outsider have informed, if not her decision to become an actress, then certainly the choices she has made as one. Talent-spotted in an all-female version of Measure for Measure at the City of London School for Girls, she was cast as a young Judi Dench in Gillies MacKinnon’s The Last of the Blonde Bombshells.

She says that the roles since then have been defined by a lot of outsiders, observers and writers, and often in period settings. All, like her perhaps, are fiercely intelligent, and yet staring coolly at life from the margins. Even in her next movie, François Ozon’s Angel, she stars opposite Charlotte Rampling as a novelist in early 20th-century London who has rejected reality in favour of her own fantasy world.

Yes, but how can she justify taking part in Dirty Dancing 2: Havana Nights? She groans at the mere mention of that stinker, a bland prequel to the original that was set in 1950s Cuba, co-starred Diego Luna, and flopped at the box office. “Let’s just say that it taught me that sometimes it’s important to lose your dignity in order to learn about how important dignity is in your life, and leave it at that, yes?”

In the meantime Garai has decided that, finally, period stories are out, and present is in. “I’m desperate to find a contemporary film with a character that’s interesting, dignified and complex,” she says. “I struggle to find those parts, and when I do, they usually go to other actresses.”

Other actresses? Aha! Not that woman again? And if so, can we not just admit, for the sake of argument, that Garai’s show-stopping turn in Atonement is actually the justified coup de grâce in the long-running casting battle between her and Knightley?

“Certainly not!” she says, full beam. “And I would be hugely, furiously, angry if that was ever suggested by anyone, and it’s certainly not something that I believe.”

All right, you have my word – it will only be in the closing sentences.

“Oh, great,” she sighs. “I can rest easy then.”

Atonement is released on Sept 7

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