The London screening of a new Disney+ movie is already running late - but it isn't John Travolta's fault.

The star of the short film in question, The Shepherd, is sitting in the audience, patiently waiting to see the story on a big screen for the first time, 30 years after he first dreamed of adapting it.

When it becomes clear that there are more people entering the screening room than there are seats, he happily gives up his chair and perches at the end of the row.

Despite taking place on Christmas Eve and being called The Shepherd, the story is not what you might expect. Set in 1957, it follows a young Royal Air Force pilot returning home for Christmas who runs into trouble when his plane suffers electrical failure.

With navigational systems down and his radio not working, he accepts his fate. But through the darkness and fog, the pilot suddenly spots another plane, flown by a man who offers to help guide him to safety.

The story is an adaptation of the 1975 novella by Frederick Forsyth - one of the other famous faces in the audience at the screening. By the time Travolta came across the book, he had coincidentally just had a near-death experience of his own when piloting a plane.

"The kismet of the project is, I actually experienced a total electrical failure, not in a Vampire but a corporate jet, over Washington DC, prior to my discovering the book," he tells journalists after the screening.

"So when I read the book, it resonated more because of this experience I'd personally had.

"I knew what it felt like to absolutely think you're going to die. Because I had two good jet engines but I had no instruments, no electric, nothing.

"And I thought it was over, just like this boy, portrayed so beautifully [by actor Ben Radcliffe]. He captured that despair when you think you're actually going to die."

The Grease actor recalls: "I had my family on board and I said, 'This is it, I can't believe I'm going to die in this plane.'

"And then, as if by a miracle, we descended to a lower altitude, I saw the Washington DC Monument and identified that Washington National Airport was right next to it and I made a landing just like [character Freddie] does in the film. So I'm reading this book saying, I've lived this."

Travolta did not discover Forsyth's book until a few years after his own brush with death.

"I had just purchased a Vampire jet just like the one in the film. I had flown it for two years and I'm doing a film in Canada and I'm at a book store, and I see a small novella with a Vampire jet on the front of it, and I said, I have to read this."

When he read the novel, Travolta says he was struck by the feeling he had experienced the events in real life.

"I instantly fell in love with this book. And it was my dream to one day make it into a film. So a couple of years later I purchased the rights to this book, but because it was right after Pulp Fiction, I was doing one movie after another.

"So after 10 years, I let it go and decided I was never going to get to do it. Then this hero [director Iain Softley] came along who had also fallen in love with it, and brought me back into the group."

When Travolta first had the idea of starring in the film more than three decades ago, he envisaged himself portraying the young pilot who runs into difficulty.

Instead, the film sees him play the older man who helps him. "I was young enough then that I could've played that part," he jokes. "But I had to wait 30 years to play the shepherd."

The film was shot largely in the UK, which explains why Travolta was spotted in Wetherspoons and Morrisons in Norfolk last April, taking selfies with fans.

Thursday's screening - overcrowded because some are here for the second showing an hour later - is the first chance for an audience to see the story come to fruition on the silver screen.

Most viewers will watch it on Disney+ when it is released on the streaming platform in December, and at 38 minutes it is the kind of snackable, family-friendly tale that could make easy viewing over the holidays.

The film also stars Millie Kent, Simon Wilson and Steven Mackintosh, while Oscar winner Alfonso Cuarón, who directed Gravity and Roma, serves as producer.

The Shepherd has previously been adapted for the stage and radio, including by the BBC. In Canada, the story has been read on radio network CBC almost every Christmas since 1979.

"One of the reasons I think it is so enduring is the genius of the story," says Softley. "It makes you question and examine what home means and what is important.

"And it goes beyond that. It's about the kind of sacred nature in a lot of religions of bringing people home and looking after the lost traveller.

"And it's self-sacrifice," he adds. "I think also at Christmas, you don't only think about your loved ones, it's a time when you think about other people who are less fortunate.

"I think it encapsulates all of that, and I think that's why people find it moving and resonant."