In 2015 three Americans made headlines around the world when they halted an armed attack on a train travelling from Amsterdam to Paris. Now Clint Eastwood has made a film of the story - although as much about the three growing up together as about the violent attack itself.
It was only three weeks before the first day of shooting for The 15:17 to Paris that the three young men at its centre realised the movie-making proposition had just changed radically.
Spencer Stone, Anthony Sadler and Alek Skarlatos had been thrilled when Clint Eastwood bought the rights to their book about the events on a Thalys high speed train in northern France in August 2015.
The friends had spoken at length with the film-maker about their schooldays in Sacramento, California. Eastwood and his team had also plotted every detail of how they stopped the Moroccan Ayoub El-Khazzani after he emerged from a toilet on the train carrying an assault rifle and a pistol with which he shot a passenger in the back.
French prosecutors later said the gunman had boarded the train with the rifle, 270 rounds of ammunition, a pistol, a bottle of petrol and a hammer. It's remarkable that no one died in the incident.
Sadler says when Eastwood got them to re-enact some of the action for the camera they thought he was just collecting material for reference. They knew the casting of actors to play them had been going on for weeks.
"But one or two things Mr Eastwood said had us looking at each other. We said, so are you suggesting we should be in the picture? And Mr Eastwood said 'Sure, why not?'"
Stone, Sadler and Skarlatos all dislike being labelled heroes. So it's ironic that the movie came about because Eastwood, now 87, gave them an award for heroism.
"It was at a ceremony the year after the attack," Stone recalls. "And afterwards we all huddled up and decided we needed to say something to him. So we went back into the greenroom and said we have this book coming out and you should make the movie! So Mr Eastwood said to send him the book and gave us his address."
The men were in their early 20s when the incident happened. Sadler points out that for years their communication before Europe had mainly been on Skype or by phone. "It was a reunion after our schooldays because to a degree we had gone our own ways.
"In the movie you see at one point we were all in the same Christian school in Sacramento. In fact I had started out in a different neighbourhood whereas Alek and Spencer knew one another better because their mothers were next-door neighbours.
"But when I came to the school I think I opened up their world a little bit and they opened up mine. When we finally decided to get together for a grand tour of Italy and Holland and Paris the old friendship just seemed to pick up the same."
The movie flashes back from 2015 to their adolescent years together, when young actors take the roles. Comparatively little of the film is on the train but the balance works well - and in the European tour and action sequences the men play themselves with an impressive lack of awkwardness.
Skarlatos says he was surprised when he realised how much screen time would be taken by their family and school background. "But now I see that was important because it explains a lot of how we worked together during the attack. We had such a history of friendship."
When it came to the acting he says it wasn't that Eastwood gave endless seminars on technique. "We discussed basics - but everyone now is carrying a camera in their pocket.
"Clint told us that when, say, we were reconstructing being in a bar in Amsterdam we should do just what we did for real. Most locations were the real thing, which helped.
"If there was a line we didn't feel was right he said just throw it out. He said be ourselves and he would do the rest. When Mr Eastwood says he'll do the rest you've just got to trust him."
After school both Stone and Skarlatos opted for military careers - Stone in the US Air Force and Skarlatos in the Oregon Army National Guard. Did that military experience influence their split-second decision to try to take down El-Khazzani when he was intent on carnage?
Stone, who moments before had been dozing, charged down the train aisle as El-Khazzani levelled an assault rifle at him.
But the weapon didn't fire, possibly because a bullet was faulty. Stone ended up with severe gashes to his neck and thumb after El-Khazzani lashed out with a box-cutter as they struggled hand-to-hand. Skarlatos and Sadler came to Stone's assistance.
"I wasn't really thinking about everyone else: I was just thinking about myself," says Stone. "I just saw that something had to be done.
"I didn't want to die and none of us wanted to die. That's what really propelled me out of my seat: I wasn't getting up thinking I needed to save the day. After a struggle I choked El-Khazzani unconscious but after that my medical training helped me save Mark Moogalian's life (the man shot in the back): he was really losing blood."
The film shows how British-born businessman Chris Norman and a member of train staff also helped to bring the attacker under control.
Stone, Sadler and Skarlatos now all have agents or managers and are trying for film and TV careers. "We would all love to jump in and play different characters," Sadler says.
They're now engaged in a new world of acting classes and auditions. Skarlatos has already taken part in Dancing with the Stars, the US version of Strictly Come Dancing.
Stone says for all of them making the film was a great experience. "Of course I hope it will be a big hit. But I also hope it makes people think.
"Yes I'd had military training - but there's not a lot you can do to prepare yourself when something like that happens. I think it will make people ask what they would do in a similar position. And why they would do it."
The 15:17 to Paris opens in the UK on 9 February.