In Clint Eastwood's The 15:17 To Paris - a thriller about the 2015 terror attack on a Paris-bound train - the director took the unusual step of having the three heroes from that day play themselves instead of hiring actors.

Chatting with The Straits Times and other press in Los Angeles recently, the Oscar-winning film-maker and the three young Americans who foiled the Aug 21 attack - 25-year-olds Spencer Stone, Anthony Sadler and Alek Skarlatos - explain their decision to re-create the dramatic events for the film, which opens in Singapore today.

Eastwood, 87, says he has always been captivated by the story and what motivated the trio to put themselves in harm's way.

The three were on holiday, travelling from Amsterdam to Paris, when a heavily armed man shot one passenger on their train and moved to open fire on the rest.

But the friends managed to tackle and restrain him. Stone - who was injured in the process - then used first aid to save the life of the bleeding passenger. For their bravery, the three received France's highest award, the Legion of Honour. They later co-wrote a 2016 book about the event, The 15:17 To Paris: The True Story Of A Terrorist, A Train And Three American Heroes.

Eastwood says: "It was a fascinating story when it was first in the newspaper. Then I met the boys a couple of years ago (at an event in) Oklahoma City when somebody asked if I would introduce them in front of an audience.

"Then I looked at their book. And I'm always interested in what makes a person do certain things in life, whether heroic or dumb - and sometimes they're both, which maybe you could say for this," he quips as the three young men laugh good-naturedly.

But even though he surrounded them with professionally trained actors - including Judy Greer and Jenna Fischer, who play Stone and Skarlatos' mothers - Eastwood instructed them not to take any acting lessons, and instead tried "to get them just to be" themselves.

"The main thing is not to lose their reality, which a professional actor can't bring," he says. "And a professional actor would hate the idea of portraying themselves because it's the hardest thing in the world to do."

First, however, the young men had to get past the shock of receiving a phone call from Eastwood - star of the iconic Dirty Harry films and a four-time Oscar-winning director, with Best Picture and Best Director statuettes for Unforgiven (1992) and Million Dollar Baby (2004).

The second shock was when he told them he wanted to make a movie about them - starring themselves.

Recalls Sadler: "Spencer rang and said: 'Guess who just called me? Clint Eastwood. He's read the book and he's interested in (making a) picture.' And I thought it couldn't get any better than that, but then he asked us to play ourselves.'"

Stone adds that it was "a huge relief" having the respected film-maker take on the adaptation. "Because I knew he was going to do our story justice and not over-dramatise it. We had a lot of trust in him going in."

Skarlatos agrees. "I had just watched Sully" - Eastwood's film about pilot Chesley "Sully" Sullenberger's 2009 emergency landing of a plane on the Hudson River - "and I knew this was his speciality lately and I knew it would not disappoint".

The film also has the passenger who was shot, Mark Moogalian, and wife Isabelle, play themselves, while actors portray younger versions of the three heroes in flashbacks to their childhood.

Eastwood and his stars hope viewers will be inspired by the story to believe they, too, could step up and save the day if the situation called for it.

Sadler says: "We hope that it shows three ordinary guys and that people can identify with us. And take inspiration from what we did to overcome obstacles in their life and know that they're capable of the extraordinary as well."

Eastwood says he "does not know what makes a hero", but that that is what the film attempts to unravel, in part by looking at the men's upbringing and the confluence of events that brought them to that train.

"Nobody grows up knowing what's going to happen to them and we live in a world today where someone can drive up to a sidewalk and try to kill you. And sometimes you stop to think about it: What would I do? And there's nothing you can do because fate is playing a part in it all.

"That's what happened with these boys. Sometimes people can do extraordinary things," says the director, whose more recent films include the hit war biopic American Sniper (2014).

For the three newly minted stars, the acting bug has bitten - all now have Hollywood agents and Stone recently moved to Los Angeles.

At the time of the attack, Skarlatos was an Oregon Army National Guard specialist, Stone an Air Force medic and Sadler, a senior at California State University in Sacramento.

Skarlatos says: "We would love to at least give it a shot. It was such a new experience for us (acting in this film) and it took a lot of growing on our part, but it was so much fun that we can't help but (want to) see where it takes us."

"It was for sure the funnest two months of my life," says Stone. "So if I can make a career out of it, then I would absolutely love that.

"And I feel like I learnt a lot more about myself and I was able to deal with some insecurities. Because you're, in the beginning, playing yourself and picking and choosing what you're trying to show to millions of people forever.

"And being able to tell myself to let that stuff go and not worry about it was good for me as a person in the long run."

Sadler says: "Mr Eastwood gave us the confidence to attempt it when we didn't think that was even in the realm of possibility. And having him mentor us through the whole process and encourage us to keep going, it kind of gave us the confidence to pursue it further afterwards, too."

• The 15:17 To Paris opens in Singapore today.