In the late 1970s, Peter Turner was a young actor living in digs in Primrose Hill. One of his fellow lodgers in the London flat was Gloria Grahame – you know, the girl who couldn’t say no in Oklahoma!, the wayward small-town floozy in It’s a Wonderful Life.

By the time she met Peter, she was playing Sadie in a production of Somerset Maugham’s Rain at the Watford Palace. “She asked to borrow a shirt,” he says. “Then she needed a fiver. It escalated from there.” The couple began an on-off affair that seemed to dwindle to nothing – until late 1981. “Tuesday 29 September,” says Peter, embarking on a tale he’s told so often it has become a kind of ritual. “I knew she was in England, but she hadn’t called.”

But then she did call, to ask if she could stay – and, as it turns out, spend her final days – at his parents’ house in Liverpool. The image of a Hollywood legend breathing her last in a tough, riot-racked northern port is so surprising, so camera-ready, that when Peter wrote his poignant, thoughtful memoir, he was besieged by offers. When he lists some of the actors who wanted to play Gloria, it tells you all you need to know about how long it has taken – 35 years – for the story to reach the screen, and how many startlingly different forms it could have taken. He starts with Joan Collins, then mentions Whoopi Goldberg, Madonna and Barbara Hershey before finally reaching Annette Bening. Jamie Bell – who plays Peter alongside Bening in the forthcoming film – wasn’t even born when David Puttnam first optioned the story.

The book, when it was first published, was about a recent memory. The film is now a period piece, the battered Liverpool it portrays having all but vanished. As if to emphasise the point, we are speaking in the Pen Factory, an elegant bar and bistro in the city. “Thirty years ago,” says Peter, “we didn’t have DVDs, let alone YouTube.” Peter had to trust his parents for an account of his LA girlfriend’s glory days – and wait for an art-house retrospective. This is one of my favourite moments in the film: the crystalline beauty of the actual black-and-white CinemaScope Gloria, peering from under her hooded eyes into the shadowy warmth of a present-day picture house.

I’m interested in how Peter managed to keep faith in the project during the years when, as he says, “It refused to get made”, almost as if “Gloria wouldn’t have it”. What really kept him going, of course, was love, the need to do justice to Gloria. Because, yes, the city has changed and the nature of fame has changed, but one thing hasn’t: how female stars are treated. Gloria was offered the lead in the 1950 film Born Yesterday. “But,” says Peter, “she wouldn’t ride unaccompanied in the limo with the then head of RKO, Howard Hughes.” So she was thrown aside.

The star was also bullied into having work done on her face and, during her divorces, highly paid industry lawyers spread hideous stories about her that tame showbiz correspondents made sure stuck. If you watch her work now, you can see an accomplished actor doing precise, effective, self-effacing work. “The only biography of her is called Suicide Blonde,” says Peter. “As if that was her. As if she was one of her characters.”

In the shadow of Harvey Weinstein, the case of Gloria Grahame – chewed up and spat out by the studios – could not be more relevant. But the film gets its unusual power from the fact that it has something to offer in place of fame and sexual oppression. In other hands, this could have been a cheery British farce about the odd-couple pairing of a working-class lad and Hollywood royalty. “Andy Capp and Mrs Mopp” is how Peter characterises such a take.

But screenwriter Matt Greenhalgh and director Paul McGuigan make the film as much about Peter’s family as about the lovers. Gloria’s death involves them all, from Mum and Dad’s once-in-a-lifetime trip to Australia being disrupted, to older brother Joe’s heroic high-speed mercy dash. The book also has pages of funny, anxious cross-dialogue about whether to call the doctor and when to call the priest as the whole family – Peter is one of nine children – rallies round. Greenhalgh tenderly preserves all this, while granting us the unexpected pleasure of watching Bell dance again.

British films often present family as the thing you have to escape if you want to fulfil yourself. Billy Liar is a good example, even if he doesn’t, in the end, get on that train to London. But here, the family is a vortex of tolerance and understanding. Peter and Gloria’s relationship is lopsided, age inappropriate and sexually confusing. She was a brittle, demanding woman. But none of that is discussed. They all just get on and do the decent thing.

And in the end, that’s what this film is about: common decency. The decency to refuse to let someone die alone. The decency that makes us great by insisting that we meet the challenge set by the needs of others. The decency that allows us to put each other back together when power has crushed us. The decency that is altogether lacking in public life. The decency that seldom appears in films and TV programmes about working-class life but is pretty much all that makes those lives possible.

In 1974, Gloria had been diagnosed with breast cancer. She had radiation treatment, changed her diet, stopped smoking and drinking, and the cancer went into remission, only to return in 1980. Peter’s book doesn’t mention breast cancer and in it Gloria seems to have no idea that she’s going to die. Does he think it was an act? “I think she had a bucket list,” says Peter, who has obviously thought about this a great deal. “And doing good work on the British stage – to join the Royal Shakespeare Company – was part of that. Her mother had been in Shakespeare on the London stage in about 1910. She had her own drama school, which was all about clear diction and so on. Gloria had a complete Shakespeare collection in that room in Primrose Hill.”

Maybe that last fling with a young actor had something of the bucket list about it, as did experiencing life at the heart of a big, chaotic family. For three decades, Peter has been keeper of the flame. When the film comes out, that will change. After this, when people think of Gloria Grahame, the story of how she spent her last days in Liverpool will be one of the first things that will come to mind – but it will be a story that stars Jamie Bell, not Peter Turner. I worry that it’s going to be hard for him.

“I just want people to remember her,” he says. They will. And there’s a chance Gloria’s story will become shorthand for every female actor who tried to put herself back together after being torn apart by Hollywood.

“I’ll tell you what was really on her bucket list,” I say.

“What?”

“Respect.”

It ought to be a basic human right. How sad that it should become a woman’s ambition – like swimming with dolphins or climbing the Eiger. “I think she wanted to know what it was like to be respected,” I say. “I think you gave her that.”

“Yeah,” says Peter. “Maybe. I hope so.”

• Film Stars Don’t Die in Liverpool is out in the UK on 16 November and in the US on 29 December.
• This article was amended on 14 November 2017. Our original picture from the musical Oklahoma! featured the actress Shirley Jones, not Gloria Grahame, owing to an agency’s captioning error.