The normality of life on the eve of a momentous event is almost impossible to convey in a movie. The morning commute on Sept. 11 in “World Trade Center,” breakfast with the family in “Deepwater Horizon” — the more a director shows us the banal, the more portentous it becomes. We can’t un-know something’s coming, and so the impact of the event has a weird way of rippling through the past.

In his new film, “The 15:17 to Paris,” director Clint Eastwood tries a different method of recapturing life before it was changed forever. The story of three American friends who helped take down a lone gunman on a European train in the summer of 2015, “The 15:17 to Paris” casts the real-life heroes themselves — Anthony Sadler, Alek Skarlatos and Spencer Stone — in the starring roles. The use of nonactors is an unexpectedly bold experiment from Eastwood, not to mention a risky commercial gambit. That makes “The 15:17 to Paris” one of the most interesting films in the director’s canon, even if it leans more toward miss than hit.

The story (written by Dorothy Blyskal and based on the trio’s book) rewinds all the way back to middle school, when the boys formed their lifelong bonds. The film’s first third is a professional affair featuring three rather good young actors as the children, Judy Greer and Jenna Fischer as two of their mothers and Thomas Lennon as an amusingly priggish principal. It jibes oddly, though, with the rest of the film, which is largely a verite travelogue through Europe in the days before the attack.

These scenes do indeed feel utterly normal, partly because the three men — now playing themselves — seem entirely unrehearsed as they wander around Italy and Amsterdam snapping selfies. Even so, they play themselves with mixed success. Sadler, a college student at the time, has a breezy demeanor; Skarlatos, then a National Guardsman, can’t quite forget the camera; Stone, the Air Force airman who led the effort against the attack, seems most at ease on-screen. Even the best of these performances, though, feels stilted. The very non-professionalism that makes the film so compelling is also what breaks its spell.

The train attack, restaged by the men and several other real-life participants (including shooting victim Mark Moogalian), unfolds with quick, blunt intensity. That powerful moment, thrillingly close to documentary or even psychodrama, suggests the movie that “The 15:17 to Paris” could have been.