The way Sharman Macdonald tells it, the story of how The Edge of Love came to be made is almost as intriguing as the film itself. To get the blitz-era tale of Dylan Thomas, his wife, his first love and his first love’s husband on to the screen took two families, a stack of air miles and several bottles of pink champagne.

It started when the film student Rebekah Gilbertson told Macdonald, a playwright and scriptwriter, about the night her grandfather, William Killick, stormed across the cliffs to the bungalow where Thomas was living with his wife and children and shot through the window with his rifle. It will come to a conclusion on Wednesday, when Macdonald walks down the red carpet in Edinburgh with her daughter Keira Knightley, who stars as Killick’s wife, Vera, and whose name got the film made.

The Glasgow-born Macdonald, who shares Keira’s cheeky smile and enviable teeth but hides them behind streaky grey hair, began working on The Edge of Love six years ago, when her daughter was working on the first Pirates of the Caribbean film. As Knightley was under 18, her mother was there to chaperone her, so she filled her time writing.

Much of the original research into Dylan Thomas and wartime life was done by the stage actor Will Knightley, Macdonald’s husband and Keira’s father. It is a family saga about the writing of a family saga.

Caitlin, Thomas’s cigarette-cadging, handstand-turning wife, is based on Knightley, but was eventually played by Sienna Miller. “Caitlin was a wonderfully alive woman, warm and earthy and vibrant, and it was Keira’s energy that fed into the character. She was 17 when I started writing it, so, although I was looking at her and watching how she reacted, I never really thought, she’s going to play this part.”

Knightley asked to read the script, loved it, and set up some meetings in LA. It was during one of these calls that Knightley accidentally cast herself as Vera. “They said to her, ‘Of course you’re going to be in it’, and she said, ‘Well, Shar’ — that’s what she calls me, she doesn’t call me Mum — ‘I couldn’t say no’.”

Macdonald has been a successful writer for decades, penning several West End theatre hits and the screenplay to The Winter Guest, the 1997 film starring Emma Thompson. But these considerable achievements are now eclipsed by her daughter’s fame.

At a time when most mothers are funding their children’s driving lessons and pulling in favours for a summer job, Knightley was attaching her name to her mother’s project and persuading Hollywood producers to take their meetings. She also wooed director John Maybury, whom she worked with on The Jacket, by sending him poetry, pink champagne and a cake.

Macdonald was already in LA, living in a flat with Knightley while she was making the action flick Domino. Rebekah Gilbertson used her husband’s air miles to fly out and join them. She brought her sleeping bag, dossed on the sofa and the pair of them drove around LA in Knightley’s car, pitching the story of Gilbertson’s grandparents, a rifle and a beer-sodden, womanising Welsh poet.

“We had such fun, we had such a giggle,” says Macdonald, who does indeed have a very ready giggle. “People were incredibly polite and immensely kind.”

Macdonald has this effect on everyone. Ten minutes into our meeting, I find myself helping her with some theatre tickets. The waiter is clearly smitten. The money men of Los Angeles were no exception and “with a few hiccups, inevitably”, it all came together. The Edge of Love will open the Edinburgh International Film Festival on Wednesday, with Knightley, Macdonald and Sienna Miller in attendance.

Did Macdonald find getting a leg-up from her 17-year-old daughter strange or uncomfortable? She looks horrified. “I thought it was wonderful. I was so complimented by that. I don’t easily let people read my stuff. She picked it up, wanted to read it, wanted to help. I thought that was extraordinary. I couldn’t have done it without her.”

With Knightley locked into playing Vera, Macdonald did take one motherly liberty. “That was when I put the songs in because I’ve always thought she could sing, but she never thought she could.”

Knightley responded by channelling Marlene Dietrich, who she reckoned had such “a ballsy, f***-off quality to her voice that it didn’t matter if she hit the right notes”. In fact, the film opens with her husky voice and a screen-filling shot of her red, gloss-drenched lips.

Tailoring a script to help your daughter find her voice is one of the upsides of having a film star and a screenwriter in the family. There are others — with her daughter worth an estimated £18m, Macdonald need never worry about paying off her student loan.

But there are also downsides, and Macdonald has stopped reading newspapers because of the incessant, and untruthful, speculation about her daughter’s weight. “I wasn’t very good at reading papers a lot anyway, but when you read things that are not true, and you know that they’re not true, then you read other things and you’ve no idea if they’re true either.”

Knightley, with her peachy skin, luminous eyes and lustrous hair, is obviously not anorexic. She is slightly built and very lean. At one point in The Edge of Love, you can count her ribs. Lying down, her breast is a nipple on a millpond. In another scene, her spine pokes out from beneath a chunky cardigan.

Macdonald, who at 57 can wear skinny jeans, a tight T-shirt and a cropped cardigan without children pointing in the street, refuses to take the genetic credit. Knightley’s physique is all her father’s work. “She has always been thin. She’s her daddy’s daughter, with his long body. Daddy was much, much thinner than Keira.”

I widen my eyes in disbelief and she grins. “Oh yes. He eventually managed to get up to a normal weight. He’s putting on weight now, he’s 62 and looks amazing. But when he was Keira’s age he had to drink milk with honey and eggs, and go training and training and training, just to be a normal weight.

“She eats like a horse. I always want to apologise because she can eat anything that she wants and she does not put on weight.

“When she was doing King Arthur, when she was still living at home, they gave me a diet sheet to help her put on weight. I looked at it and thought, if she follows that she’s going to lose weight. It’s less than she eats normally. But they must know what they’re doing, so we followed it.”

She shakes her head. “The weight was dropping off her. I had to say, ‘Right, just eat what you did before, and start drinking protein shakes as well’.”

Actually, it turns out that it’s not all Daddy’s fault. Macdonald unbuttons her cardigan to show me her own, very modest, bustline. “Do I have breasts?” She has a point. Most Scottish men have bigger tits than Macdonald. She smiles triumphantly. “Exactly. It’s just the way she is.”

When they have finished comparing Knightley to an ironing board, the snipers usually turn their attention to Knightley’s acting abilities.

Her mother, who acted herself before stage fright forced a switch to writing, is having none of it. “I don’t believe them for a moment.

“She’s brilliant, absolutely brilliant. I think she’s wonderful in The Edge of Love. I thought she was wonderful in Bend It Like Beckham. She’s growing up, she has more experience. And the role is rich. Your performance can only be as good as the material you are given. She is an amazing talent. And I’ve seen her on stage.”

As the rumour mill currently has Knightley in negotiation with Cameron Mackintosh to appear as Eliza Doolittle in a West End production of My Fair Lady, Macdonald quickly realises that this is probably not something she should be talking about. But I try anyway. Does Keira want to do more stage work? Her mother throws up her hands. “Who knows?”

As for Macdonald’s own future, she is finding it hard to see past the premiere next week. She has several scripts on the go, including one about Ernest Hemingway and Martha Gellhorn, but years of writing and acting have left her with a superstitious dread of jinxing a project by talking about it too early. Her tiny feet — small enough to borrow Knightley’s Christian Louboutins — are firmly on the ground.

“If you go into this business you don’t think it’s going to work,” she says, grinning, looking incredibly like her daughter.

“Whatever you get has to be enough for you. You just keep taking it as it comes.”

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