LOS ANGELES –  Clint Eastwood remembers being stunned and then gratified on Aug. 21, 2015, when he heard that three Americans had thwarted a terrorist brandishing an AK-47 on a crowded train to Paris.

Former National Guardsman Alek Skarlatos, left, Anthony Sadler and former US Air Force staff sergeant Spencer Stone arrive for the world premiere of "The 15:17 to Paris." These real-life heroes play themselves in the film.
Chris Delmas/AFP/Getty Images

"I had this great sense of pride. That was a great event they pulled off," Eastwood recalls of Alek Skarlatos, Spencer Stone and Anthony Sadler, who stopped attacker Ayoub El-Khazzani. "This guy had an AK-47 and somewhere between 250 and 300 rounds of ammunition, pistols and knives. He could've killed 300 people, easy.

"I thought, 'I'd like to meet these guys,' " he adds.

More: How accurate is the heroic fight in Clint Eastwood's 'The 15:17 to Paris'?

Also: Three men who thwarted terror attack to star in Clint Eastwood film

Eastwood did meet the three heroes — childhood friends traveling around Europe together — backstage in 2016 after presenting them with a Spike TV Guys Choice Award for their bravery.

Director Clint Eastwood (center) found real-life stars for 'The 15:17 to Paris' in (from left) Anthony Sadler, Spencer Stone and Alek Skarlatos.
Robert Hanashiro/USA TODAY

The meeting was momentous, not only because it triggered Eastwood's work on his film centered around the high-speed train ride, The 15:17 to Paris (in theaters Friday). But the director, 87, also eventually stopped his casting process and asked the charismatic Americans to play themselves onscreen, despite their having no acting experience. 

Even the guys, who wrote the book that the movie is based upon, were stunned listening to Eastwood's proposal in his office weeks before shooting.

"What went through our heads was this was a lot of pressure. We don’t want to disappoint. He’s taking a big risk by doing this and we’re taking a big risk," says Skarlatos, who appeared on Season 21 of Dancing With the Stars. "We don’t want to ruin our life story, either. We did have a little debate. But you cannot say 'no' to an opportunity like that."

"Besides, everybody knocks out a flop every now and then," Eastwood adds wryly.

Spencer Stone, left, Anthony Sadler and director Clint Eastwood on the set of 'The 15:17 to Paris.'
Keith Bernstein/Warner Bros.

In reality, Eastwood was comfortable taking the major gamble of casting three unknown quantities in his lead roles. 

"I never looked back. But I did think about, 'What happens if these guys turn out to be three stiffs?' " says Eastwood, cracking his stars up during a group interview at the Four Seasons Hotel. "I had a period in my life where I spent more time thinking. But that thinking can get you in a lot of trouble. Sometimes when you get a good idea, you have to take it."

His stars were naturals portraying themselves, re-creating the European jaunt they took while Stone and Skarlatos were on leave from their duties in the Air Force and Oregon National Guard, respectively.

The casting proved even more powerful when the director restaged the fateful attack on the same French train line.

Spencer Stone (from left), Alek Skarlatos and Anthony Sadler on board 'The 15:17 to Paris.'
Keith Bernstein/Warner Bros

"When we were on the train, it was the real deal," says Eastwood, looking at his stars. "An actor tries to get into the character to that degree. The fact is, you were playing yourselves. We just kept it going."

The trio felt a sense of catharsis as they filmed the battle. Eastwood hopes experiencing it onscreen will help others realize just how profound that moment was.

"I don’t know what Clint Eastwood would have done. No one knows what you’re going to do in that situation unless you have lived it," says Eastwood. "And these three fellas have."