At the programme launch of the 2008 Edinburgh International Film Festival artistic director Hannah McGill thanked the makers of The Edge of Love for providing her with the perfect opening night film. What McGill no doubt meant by this was that The Edge of Love is a very fine film, but moreover one that boasts a talented, young cast in Keira Knightley, Sienna Miller, Cillian Murphy and Matthew Rhys, directed by John Love is the Devil Maybury, and with an excellent script by Glasgow-born playwright and screenwriter Sharman Macdonald, who also happens to be Knightley’s mother.

Under McGill’s directorship, the EIFF has focused on writing and women, so the film perfectly suits the festival’s opening night slot. It takes as its subject matter the ménage-à-quartre between poet Dylan Thomas, his wife Caitlin, his childhood sweetheart Vera Phillips and Phillips’ husband William Killick in wartime London and rural Wales. What’s particularly appealing about the script is the way it eschews the conventions of the traditional artist biopic – foregrounding of the male and his personal demons and the relegation of the female muse to the background – and instead focuses on the two women’s relationship, and the relationships between all four characters.

On the way to a script meeting in Paris, Sharman Macdonald stopped off in London to talk about The Edge of Love. Whizzing from too much coffee, and excited about popping over to France on the Eurostar, Macdonald, a lovely, chatty bohemian who evidently passed on her good looks to her daughter, confirms that she set out to write an unconventional artist biopic. ‘I was never interested in writing a biopic,’ she says, dumping her travel bags to share her second pot of coffee of the day. ‘I find it awfully difficult to look at whole lives. Dylan Thomas died in 1953, reportedly after drinking 18 whiskies. In our film, he hasn’t even written Under Milk Wood by the time it ends. It’s a story about four friends, and about the borders between love and sex and friendship and what happens when they’re crossed. Two of these people happen to be Dylan and Caitlin Thomas. Of course, realistically there would never have been a film made had two of the characters not happened to be Dylan and Caitlin Thomas.’

Macdonald says she wrote the first draft of The Edge of Love six years ago, after she was approached by Rebecca Gilbertson, granddaughter of the Killicks, who became one of the film’s producers. ‘I started writing the film when Keira was making the first Pirates of the Caribbean,’ Macdonald says. ‘I was with her [on location] on St Vincent, but I was actually very reluctant to do it. I owe a huge debt of gratitude to my husband [actor Will Knightley], because he did a lot of the research and a story dateline breakdown. I was working on three other projects at the time, and I just thought, “I can’t take on any more.” I wrote the film in so many places, mostly abroad: Los Angeles, the Bahamas, New York, France. I actually think it was a really good thing to write from a distance. I write about people in Scotland from my home in England, so I guess I’ve always written from a distance.’

Macdonald wrote the part of Caitlin with her daughter in mind. As it turned out, Knightley was instrumental in getting The Edge of Love greenlit, and although she did take a role in the film, it wasn’t that of Caitlin Thomas but of Vera Phillips.

‘She was only 17 when I started writing it, so she was far too young to play either part,’ says Macdonald. ‘But I’ve always been fascinated by people’s energies. When I wrote Caitlin, I was writing about Keira’s energies as a 17-year-old and the part was coming off the page so easily for me, whereas Vera was harder, because she was a more passive character. Two years later, Keira picked up the script, when she was working on Domino and said, “Can I help get this made? I’ll see if I can get meetings for you.” She did, and afterwards said, “The studio asked me if I was going to be in it. I couldn’t say no, could I?” So I said to Keira, “Right. So you’re playing Caitlin?” She went, “No, Vera.”’

‘Now,’ Macdonald says, ‘I like passive central characters, because they reflect what’s going on around them. But that doesn’t necessarily make them a joy to play, although, oddly, that’s what attracted Keira to the part. You need something else as a performer, so I pushed the character and that was when Vera became a singer.’

The Edge of Love opens with Vera singing to the troops in a makeshift nightclub in a London Underground station during the Blitz. The grim reality of the situation is subverted by Maybury’s stylish treatment of the scene, which is intended to be filtered through Vera’s imagination. It was, reportedly, a stretch for Knightley to sing for the film, but she’s sensational in the scene. Macdonald is very happy with all the performances.

‘The boys are to die for,’ Macdonald says. ‘Sienna’s amazing, Keira’s amazing, and they’re amazing together. Keira’s a quarter Welsh, and she has an innate Celtic quality in the playing of the part. It’s a warmth and a brittleness and a sensuality. Sienna’s so earthy, and incredibly sexy. Whereas Keira is sensual, which makes for a wonderful chemistry between them.’

There’s a nice footnote to this story. In a way, Macdonald helping her daughter’s career along by writing her a great part is payback, because Keira was instrumental in her mother becoming a writer in the first place. Back in 1984, Macdonald was an actor, but she’d developed debilitating stage fright and wanted to quit. ‘I was also desperate for another child when my son Caleb was four or five,’ she says, ‘and so my husband made a deal: the sale of a script for the birth of a child. So I wrote When I Was a Girl, I Used to Scream and Shout [which won the Evening Standard Award for best newcomer] and the next year Keira was born. And once the play went to the West End I then thought, “Shit! I’ve got to do this for the rest of my life.”’

The Edge of Love, Cineworld, Edinburgh, Wed 18 Jun, 9.30pm & 9.45pm, £11 (£8.80).