Two English roses, in the unmistakeable forms of Keira Knightley and Sienna Miller, a handsome Welsh anti-hero, played by Matthew Rhys, and a salivating press pack from all around the world. For centuries impregnable, Edinburgh Castle was well and truly stormed to mark the opening of the city's international film festival.

This was a media conference to mark the world premiere of the festival's opening film, The Edge of Love. Described by its director John Maybury as a “feelgood chick-flick”, it is a romantic drama about the friendship between Dylan Thomas and his wife Caitlin, and the poet's friend Vera Phillips and her husband, William Killick.

The film features verse, and much worse: brutality, drunkenness, violence, sex and betrayal. All in all, producing the kind of atmosphere which can enervate fans and news cameramen, but which can, after a while, weary the stars.

Through much of this trial by media, Miller seemed to be leaning on Maybury for support, perhaps fearing an attack of the paparazzi who have so often plagued her life. But she needn't have worried. All eyes here were on Knightley, cast as Vera, the poet's lover, who takes her clothes off and has sex with Thomas on screen. And this in a film scripted by the actress's mother, Sharman MacDonald.

Grey-haired and smiling, MacDonald - or “Shar”, as Knightley calls her - had already said that she never went near the set when her daughter was kissing or simulating sex. So did it feel odd for the actress to play out these same scenes which had been contrived by own her dear parent?

“I don't have a problem with that,” snorted Knightley, whose father is Will Knightley, the actor. “It's a story about relationships, about how friendship can be ruined over an absolute betrayal in the form of sex. I can't really get out of the fact that there is sex involved in it. It certainly didn't worry me that my mother actually knows what sex is.”

Another questioner ventured into the same territory. The scene would surely capture the imagination of many of Knightley's fans. And it would put to rest all that nonsensical speculation about a naturally slim actress who has had to put up with many unkind remarks about body image.

“No. It was very simple,” retorted Knightley. “It was a sex scene and I never like them when they've got bras on. John Maybury said, ‘Take your bra off,' and I said, ‘All right then'.”

Prurience aside, The Edge of Love is an evocation of four lives which are buffeted in war. It is co-produced by Rebekah Gilbertson, whose grandparents were the couple who featured in Thomases' life in the early 1940s, first in London and then in South Wales.

Philips had known Thomas as a child in Swansea, and met him again much later by which time she had married Killick, an army commando, who had been psychologically damaged in battle. One evening, after a row in a pub, Killick fired shots into the Thomases' house, kicked down the front door and then produced a hand grenade, before he was dissuaded from the ultimate act of destruction. He was later tried for attempted murder.

Thomas aficionados will have to suspend their disbelief for parts of the film. MacDonald admitted taking “enormous liberties” with these four people, and could not prove that there had been betrayals in love. “I do not know what the truth is. I don't think anyone left alive does,” she said.

Then there is the matter of casting. The poet “had a pink blubbery face with pop eyes, lose lips and a tadpole body” according to Caitlin herself, but is played here by the chisel-jawed Rhys. Caitlin, statuesque in life, could reputedly carry her husband “across streams” under one arm; here she is brought to life by the elfin Miller.

MacDonald had initially imagined her daughter as Caitlin, but Knightley demurred. “When I first read the script I completely fell in love with Vera, her subtlety, her quiet, not very dramatic way. I felt her emotions very keenly,” said the actress.

Her mother had her “revenge” on her daughter for not taking the part. As Vera, she was obliged to sing on set. That had been an ordeal, admitted Knightley. “My knees started buckling. Then someone very kindly brought me a couple of shots of vodka and everything was alright.”

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