Interview: Keira Knightley hits a high note in her latest role

Keira Knightley's new role is as a singer who is plunged into a complex love affair with Dylan Thomas. John Walsh talks to an actress who'll try anything once

The scene is an Underground station during the Blitz. Outside, Luftwaffe shells may be raining down on the London streets but inside, among the huddled masses sheltering in the mole-y gloom, something equally explosive is about to happen. Someone has rigged up a band-leader's microphone and towards it come the pillarbox-red lips of the most beautiful face you've ever seen on a Tube platform. Keira Knightley's astounding physiognomy is framed by marcelled waves of chestnut hair. Blue fingers of ferny frou-frou tickle her alabaster flesh. A gardenia fascinator is clamped to her head like a crown.

She bends to the microphone and, breathily and sleepy-eyed, she sings "Underneath a Blue Tahitian Moon". Within 20 seconds, every male heart on the platform (and in the cinema) becomes her devoted slave, as her eyes and lips and hair and skin and voice construct a sensory web of enchantment.

Keira Knightley can sing? As well as play football (Bend it Like Beckham), wield a sword (Pirates of the Caribbean 2), fight with a Chinese jian (Pirates 3,) dominate a shower-room of steaming jocks while wearing school uniform (The Hole), rise from under water in an ornamental pond looking far more naked than if she were actually naked (Atonement), and pick up a Best Actress Oscar nomination at only 20 (Pride and Prejudice)? As revelations go, this one is up there with the shout-line "Garbo Talks!" on the posters for Anna Christie in 1930. But it would be wrong if the film that opens with this coup de théâtre were lauded for the chanteuse stuff alone.

The Edge of Love is a remarkable piece of work, a sublimely emotional wartime drama, an intense four-hander ensemble piece with a shoot-'em-up climax of gunfire and murder trial. The story concerns Dylan Thomas, the legendary Welsh poet, his London-Irish wife, Caitlin, his childhood friend Vera Williams and her romance and marriage with a British soldier called William Killick. Dylan and Vera meet each other in London in 1940, and an intense friendship is kindled between the wife and the former playmate.

When Vera marries William, the two couples occupy adjacent houses on the cliffs of New Quay in Cardiganshire, in blissful harmony. But when the soldier goes off to war, the others' lives hit a downward spiral of penury, frustration and jealousy – and when William returns, a damaged and suspicious stranger, his resentment of the smugly lecherous Dylan leads to mayhem. William is played, with clipped precision, by the Irish actor Cillian Murphy; Matthew Rhys portrays Thomas as a sleepy-eyed booze-hound, but makes him a lot sexier (and less irritating) than history reports. Sienna Miller plays Caitlin as a hoydenish "free spirit" and chronic sexpot. And Knightley gives Vera an independence and complexity that's aeons ahead of the spunky pirate babe Elizabeth Swann or the crosspatch aristocrat Cecilia Tallis in Atonement.

A curious detail about the film is that Vera's character changed as the screenplay evolved. It was written by the playwright Sharman Macdonald, Keira's mother, after she heard the story of Vera and William from their granddaughter Rebekah Gilbertson, who produced The Edge of Love. "When I was writing it, I always had Keira in mind for Caitlin Thomas," says Macdonald, "because she has Caitlin's spirit. Then she read it and said, 'I want to do this, I want to help it get made.' I said, 'Yes, and you're playing Caitlin,' and she replied, 'No, I like Vera.'" .........

That's daughters for you. "I don't know why I wanted to play her instead," says Knightley. "But the first time I read the script, I was incredibly moved by Vera. She was a much quieter character than Caitlin, and I love the way she becomes stripped down, completely tragic and beautiful." While Vera sings and dreams of stardom ("I think she secretly wanted to be Veronica Lake," says Knightley, "an ultra-glamorous star"), her fortunes wane and her beauty fades – or as much as any make-up department can de-glamourise the divine actress. "There's one scene when she looks in a mirror and suddenly realises she's not the person she was. It's a very real and honest moment. She was trying to get back to the person she was when they met and fell in love, and failing completely."

Was it difficult for her, at 23, to play an emotional wreck? "But I don't think of her as a wreck," said Knightley reprovingly. "It's more subtle than that. It's about the passing of time and the changing of one's outlook. I loved how quietly you see her changing." Ironically, once the daughter signed up to the role, her mother set about altering it.

"In the first couple of drafts, hers was a very passive role," says Knightley. "I think Shar [she always calls her mother 'Shar'] felt the balance of the relationship weighed towards Caitlin because she's such a vivacious character, so she beefed up Vera to make them equals. She gave Vera a bit more bite."

And, of course, she made her a singer. Knightley pulls a face when asked about her debut as a crooner. "My mum has always believed I can sing, and I've always told her I can't. But she said, if I was going to play Vera, then I was bloody well singing." Knightley briefly regressed into a sulky teen. "Yeah, it makes her a more interesting character, that dynamic of desperately wanting to be a singer and loving the glamour side and all that, but I found it fucking terrifying. But it looks good and that's the main thing."

If it bothered her, couldn't another singer's voice have been dubbed? The film's director, John Maybury, who brilliantly evoked the arty demi-monde of Francis Bacon in Love is the Devil, came to her aid. "He told me, 'Look – Vera is singing in a Tube station. She's not a Hollywood star with a great voice. She's not meant to be that good – the worse she sounds, the better.' So I said, 'Oh, great – if I hit a wrong note, I can say it was a character choice.'"

When the moment came for the Tube scene, Knightley was expecting to mime to her pre-recorded voice, but was told by Maybury to sing live, in front of 100 extras and crew. "I was shaking like a leaf, I thought my knees were going to buckle. In the first couple of songs, I sounded like a pubescent boy, it was so embarrassing, I could see all these faces thinking, Oh my God, isn't she terrible. But then I had a couple of shots of vodka and it was much better."

When I ask if she'd been watching footage of Vera Lynn or The Andrews Sisters, to see how wartime crooners acted, she becomes defensive, like a schoolgirl accused of copying her homework.

"If I tell you who I listened to, it's only because it was nice to have them in my head, I wasn't trying to do an impression. I watched Marlene Dietrich films and she was an influence but only in the way she stands there and doesn't hit a single note. I watched the way she smoked and the way she and Garbo used cigarettes. My problem is, I always give away my influences and people say, 'Well that was a shit Marlene impression,' but that's not what it was at all..."

It's a remarkably physical movie, crammed with claspings and sunderings, the twang of bedsprings, the touching of hands. Dead bodies on the battlefield or the aftermath of London bombs are mirrored by the horizontalism of lovers and the girlish embraces of Knightley and Miller who spend a lot of screen-time sharing a bed for warmth and larks. The actresses' on-screen rapport is very believable. Was it just good acting?

"I think warmth is one of the hardest things to fake," says Knightley. "You can fake pretty much anything, but not that. We were fortunate that all four of us got on, particularly me and Sienna. I really enjoyed spending time with her, and I don't think we could have created the friendship as well as we did if it hadn't been there in real life."

She wasn't impressed by internet rumours that the film would feature some hot girl-on-girl lesbionic action by the two A-listers in the sack.

"Funny, isn't it?" says Knightley, sadly, "how as soon as you have a film that centres on female friendship, everyone thinks it must have a sexual nature, whereas if you have two men in a film about male friendship, it's fine, it's a buddy movie, I think that says something about our society in general."

Some of Knightley's best scenes are those shared with Matthew Rhys as Dylan Thomas. Even as she prepares to marry William Killick, and the four friends celebrate and dance together, Vera and her childhood sweetheart exchange secret glances that may conceal old intimacies or hint at future ones.

In real life, Vera was ambivalent about the poet. "I had more fun with Dylan, and with Catty too, than with anybody else, before or since," she told a biographer. "He was such a selfish little bastard, it was surprising to find him married to someone it was so easy to get on with." So did Knightley think Vera was in love with him? "I've decided she was. He was part of her childhood and you always romanticise your childhood. I think she was in love with his talent and his mischief and who they were when they were small."

The producers, I point out, and the screenwriter think they've left ambiguous the crucial detail of whether Vera cheated on her husband with Dylan. "You think that's unclear?" asks Knightly incredulously. "After he pulls her into the bathwater with him?"

It doesn't necessarily mean, I point out, that sexual congress ensued. "I very much assumed," said Keira, with the air of one who knows, "that it meant something certainly did happen shortly afterwards." Well, our social lives must be very different.

The immediate future holds a second movie release this September – Saul Dibb's The Duchess, based on Amanda Foreman's sparkling biography of Georgiana, Duchess of Devonshire, which will see Knightley in a Marie Antoinette wig and a ton of rouge, married to the brutish Ralph Fiennes and failing to keep the hands of young Dominic Cooper out of her foundation garments. After that, she has nothing planned. Will she court more serious Edge of Love-style parts in future, or be seduced by more adventures with brigands and corsetry? "My options are completely open," she says firmly. "I'm not a snob about pure entertainment. I love doing films that are challenging and provoke discussion. And I love films where you can let it all wash over you and cry at the end. My aim, if I have an aim, is to try as many genres as possible."

Smart girl. And in the unlikely event that the phone doesn't ring, she can always make an album called Keira Knightley Sings Vera Lynn....