Grasping for a way to describe the indescribable, stunned survivors of terror attacks have often reached for the imagery of cinema: “It was like a movie.” But that phrase has never taken on quite the same meaning as it has for Spencer Stone, Alek Skarlatos and Anthony Sadler.

On Aug. 21, 2015, the three young American friends were on a backpacking trip through Europe when they thwarted a terror attack on a high-speed Thalys train from Amsterdam to Paris, tackling and subduing Moroccan-born Ayoub El Khazzani, who was armed with an AK-47 assault rifle, a 9mm pistol and other weapons.

Their bravery made headlines around the world and earned France’s Legion of Honour, various military awards and a visit to the White House.

Now, in the latest improbable twist, the three are playing themselves in a Clint Eastwood-directed film about the event, “The 15:17 to Paris."

On a recent afternoon, Stone, Skarlatos and Sadler — who are all 25 — sat in a hotel lobby in Burbank after a taping of “The Ellen DeGeneres Show,” trying to wrap their heads around this surreal turn of events. Childhood friends who grew up together in Sacramento, the three have never been in so much as a school play. Yet here they are starring in a major studio film directed by a genuine Hollywood legend.

“We were raised on Clint Eastwood’s movies — I still remember at Spencer’s house he had ‘Hang ‘Em High,’ ‘The Good, the Bad and the Ugly,’" said Skarlatos, who was an Oregon Army National Guard specialist at the time of the attack, on holiday after a stint in Afghanistan. He shook his head. “To get to do all this with guys that you’ve been friends with for so long — our whole lives have been like a movie. It’s ridiculous.”

Eastwood first met Stone, Skarlatos and Sadler when he presented them with the Hero Award at the 2016 Spike Guy’s Choice Awards. Agreeing to direct “The 15:17 to Paris” — which retraces the events leading up to the attack all the way back to the three men’s childhoods — he had spent weeks auditioning actors to play them. But in his mind, he kept circling back to the real-life guys.

“I thought they were very charismatic, and all three of them seemed like they were extremely smart,” Eastwood said. “I thought, ‘If I can get them approaching this thing without too much thought and too much worry and anxiety, they could do it.’”

Casting people as themselves in movies is not unprecedented, of course. Decorated World War II soldier Audie Murphy played himself in the 1955 film “To Hell and Back.” Jackie Robinson and Muhammad Ali starred in their own biopics, as did Howard Stern. Many more real-life figures have popped up in small cameos in film versions of their stories.

Still, when Eastwood first asked Stone, Skarlatos and Sadler if they’d be up for playing themselves, they weren’t sure how to respond. Though they had co-authored the 2016 book on which the film was based with Jeffrey E. Stern, the idea of actually appearing in the movie had never occurred to them. The start of production was just three weeks away, and none of them had ever even set foot on a movie set.

They said yes, then almost immediately began to second-guess themselves.

“I really didn’t want to ruin the movie,” Sadler said. “I’m like, ‘Actors can do this and it would probably be more successful.’ But Spencer was like, ‘Are you really going to look 20 years down the line and say you could have been in a Clint Eastwood movie but you’re not?’ And that convinced me right there. There’s no way you could deny that.”

With what little time they had before shooting, the three wanted to prepare in any way they could. “When the reality set in, we were like, ‘We’re really thankful for the opportunity, Mr. Eastwood, but we also think we’re going to need some acting classes,’" said Stone, who was an Air Force medic at the time of the attack. “And he was like, ‘No, you don’t want to do that, because then it will make it look like you’re acting. I just want you to go out there and be natural and do it how it happened.’"

Eastwood admits that the prospect of casting nonactors didn’t thrill the executives at Warner Bros.

“I don’t think they were excited at the beginning,” he said with a dry laugh.

But in a testament to the tremendous amount of clout Eastwood has at the studio — where his production company is based and where he has directed such films as “Mystic River,” “Million Dollar Baby,” “American Sniper” and “Sully” — they agreed to go along with the idea.

There were some bumpy moments at first. In his first big scene, set in a Jamba Juice, Stone “felt like I was crashing and burning.” In another scene, Sadler, unaware of the need for continuity, kept picking up different things with his hands in different takes.

But things soon slipped into gear, and the studio was reassured that the experiment could work. “I think — or at least I hope — that they were pleasantly surprised,” Eastwood said.

The attack itself was re-created over five days on a moving train traveling back and forth between Paris and Amsterdam. Along with Stone, Skarlatos and Sadler, a number of other real-life participants were also on hand, including Mark Moogalian, who had sustained a gunshot injury to the neck, and English businessman Chris Norman, who had helped restrain the attacker with a necktie and some cable.

Though the actual attack lasted only about two minutes, Eastwood was intent on re-creating it with exacting, second-by-second fealty, with no extra Hollywood action-movie bells and whistles. “I didn’t want to make it look like guys were hanging from the bottom of a train and doing crazy things,” Eastwood said. In his mind, what transpired in those two minutes was dramatic enough.

Moments after the attack had begun, Stone — who had been awakened from a nap by sounds of gunfire and screaming — ran straight at El Khazzani and took him down. In a stroke of luck, the attacker’s AK-47 had jammed and he had accidentally dropped the magazine from his pistol. (“We got the world’s worst terrorist,” Skarlatos said dryly.)

In the ensuing struggle, Stone sustained gashes to his neck and thumb with a box cutter, putting El Khazzani in a chokehold as Skarlatos disarmed him and bashed him in the head with the butt of his rifle, knocking him unconscious. Drawing on his medical training, Stone then worked to stop the bleeding in Moogalian’s neck.

“That was an intense flashback,” said Stone of re-creating that moment onscreen. “We’re wearing the same clothes. It’s Mark. It’s his wife. Everything was exactly the same.”

For Eastwood, the film hinges on the question of what motivated the three to move toward danger in that critical moment. “What would most people do — especially me? You’d hide under the seat,” Eastwood said. “What makes a person just jump up and run at somebody who has a very reliable weapon pointed at them?”

Though Eastwood insists that the film has no political bent, its story of everyday Americans foiling a terror attack seems likely to strike a similar chord with some audiences as 2014’s “American Sniper.” That film grossed $547 million worldwide, easily the biggest hit of Eastwood’s career.

But as for what it will mean to moviegoers to watch the actual participants rather than actors re-creating that story, Eastwood isn’t certain.

“I don’t know,” he said. “There’s certainly a curiosity about it, at least from my experience of people on the street. They seem to think it’s an interesting idea. But we’ll see.”

For their part, Stone, Skarlatos and Sadler still speak of their time with Eastwood with a sense of awe. “We worked out with Clint Eastwood in Venice, and this dude did 10 body-weight dips,” marveled Skarlatos, who was a contestant on Season 21 of “Dancing With the Stars,” finishing in third place. “87 years old! It was the coolest thing I’ve ever seen.”

Having gotten a taste of Hollywood, the three all now aim to pursue acting professionally. Stone has already moved to Los Angeles and Sadler is soon to follow. Acting classes have been taken. Agents and publicists have been retained.

“I’ll never forget: One day between takes Clint Eastwood looks at me and he’s like, ‘Not a bad way to make a living, is it?’ “ Sadler said. “I was like, ‘No, you’re right. It’s not a bad way to make a living.’ We’d all like to pursue it for sure.”

That said, he added, “If it ends now, it would be enough. And even if all of the world hated the movie, how could we hate it? This is what happened.”

If you go

THE 15:17 TO PARIS

PG-13, 94 minutes

Playing at Waterford, Stonington, Waterford, Westbrook, Lisbon