Clint Eastwood has revealed why he decided to cast real-life heroes in his upcoming terrorism thriller, The 15:17 to Paris.

The 15:17 to Paris is based on the true story of three American friends, Spencer Stone, Alek Skarlatos and Anthony Sadler, who prevented a terrorist attack on board a French train in 2015.

Eastwood Explains Casting Decision

Eastwood has previously helmed films based on real-life heroes, including American Sniper, starring Bradley Cooper as Navy SEAL Chris Kyle, and Sully, featuring Tom Hanks as heroic airline pilot Capt. Chesley Sullenberger.

However, Eastwood's upcoming film, which is inspired by a book penned by the three Americans, took many by surprise when it was announced that after a lengthy search, Eastwood was going to cast the real-life heroes in his film.

In a recent interview with The Hollywood Reporter, the director revealed the reason behind his unconventional decision to choose the non-actors for the film.

The director said he was going through the details of the Thalys train attack with Stone, Skarlatos, and Sadler repeatedly to make sure the events were accurately depicted in the film and, one day, it just hit him that the trio would be a perfect fit for the role.

"I think there are some wonderful actors around that could've played this, but there's something about this particular project and the heroism that was involved and the way they handled the thing that is just kind of unique, so I thought I'd try that here. And I just said, 'I think I'll take a shot at it,'" he added.

The 15:17 to Paris

Stone, Skarlatos, and Sadler will play themselves in the film, which will document the events that took place on a Thalys train traveling from Amsterdam to Paris on Aug. 21, 2015.

The 15:17 to Paris will tell the backstory of the three friends who grew up together in Sacramento right from their childhood to the incident that made them American heroes.

The three tourists saved countless lives by overpowering and subduing an armed ISIS gunman carrying more than 300 rounds of ammunition, who opened fire on the train.

Two other men who helped thwart the attack, British national Chris Norman and Armenian-American professor Mark Moogalian, will also play themselves in the upcoming film. In the attack, Norman was stabbed several times in the neck while Moogalian was shot in the back.

The 15:17 to Paris is slated to hit theaters on Friday, Feb. 9. Check out the film's trailer below.