In many a Mel Gibson movie, there comes a crucial moment when a peace-loving man must pick up a weapon. In “Braveheart” it was a 13th-century Scottish sword; in “Apocalypto” it was a tribal spear. In Gibson’s newest film, the World War II drama “Hacksaw Ridge,” you might assume the weapon would be a government-issued rifle.

That crucial moment, however, never comes in “Hacksaw Ridge” (opening Friday), which stars Andrew Garfield as the real-life pacifist soldier Desmond Doss. A skinny kid from Virginia, Doss served in the Army but refused to touch a weapon, citing his Seventh-day Adventist beliefs. Despite mockery from his fellow soldiers and a near court-martial from his superiors, Doss served as a medic on the front lines of Okinawa and wound up saving the lives of a reported 75 men left for dead atop a 350-foot escarpment known as Hacksaw Ridge. Doss would go on to be awarded the Medal of Honor for “outstanding bravery and unflinching determination in the face of desperately dangerous conditions.”

“Hacksaw Ridge” isn’t the first time Gibson has made a movie about a pacifist — that would be “The Passion of the Christ” in 2004 — but it seems to mark a change of pace for an actor-director who typically traffics in action, violence and bloodshed. In such movies as “Mad Max,” “The Patriot” and “Payback,” Gibson has been drawn to heroes who suffer and brutalize in return. Off-screen, too, Gibson has earned a reputation for volatility after his 2006 DUI arrest and a spate of outbursts directed at gays and Jews, among others. “Hacksaw Ridge” marks Gibson’s first film as a director since that dark period, but he doesn’t see it as a radical departure.

“Not at all,” the director says, sounding almost surprised by the question. “It’s kind of an evolution of an idea I work with. Basically, it’s the ultimate hero story, even more heroic for the fact that he doesn’t engage in armed conflict. For me, he transcends the hell of war.”

Although “Hacksaw Ridge” is rated R for its graphic and highly realistic war scenes, Gibson goes so far as to say it isn’t really a war movie. “It’s a love story,” he says.

KEPT A LOW PROFILE

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Doss’ heroics may seem tailor-made for Hollywood, but by all accounts he was a private man uninterested in capitalizing on his story. Only near the end of his life did he give his blessing to a documentary by Terry Benedict, released in 2004, titled “The Conscientious Objector.” (Doss reportedly disliked that term; in Gibson’s film, the Doss character calls himself a “conscientious co-operator”). Doss died in 2006, at the age of 87, just months before the playwright Richard Shenkkan was approached to begin work on a screenplay.

“To be honest, I didn’t know the Desmond Doss story,” says Shenkkan, who became one of the film’s two writers. In terms of immediate research material, he says, all he had was the documentary, one newspaper article and — oddly enough — a comic book sanctioned by the Congressional Medal of Honor Society. “And on the basis of that, I said yes.” (There’s also a 1967 biography, “The Unlikeliest Hero,” by Booton Herndon, among a few books about Doss.)

When the film’s producer, Bill Mechanic — an industry veteran who now runs his own company, Pandemonium Films — asked for suggestions about who should direct, Shenkkan says he immediately nominated Gibson.

“When people think of Mel, the knee-jerk reaction is: He’s a great action director who knows his way around the world of men and violence. But Mel, I think, has always been extraordinarily adept with actors and very good at pulling out really complex human emotions. And I think that’s exactly what he did here.”

For Garfield, playing a soft-spoken man “who didn’t want a movie to be made about him” was a challenge. “You could call him an introvert, but I’m not sure that fully describes him,” says Garfield. “He embodied the credo ‘Live and let live.’ There was no point where he was trying to foist his ideas on anyone. He knew he couldn’t intervene with your faith, but he also knew that nobody else could intervene in his.”

A DIFFICULT SCENE

Gibson and Garfield, speaking together by phone from a Los Angeles hotel room, admit they sometimes felt frustrated by their stubbornly nonviolent hero. As an example, Gibson points to a tense scene in which a fellow soldier, Smitty (Luke Bracey), taunts and strikes Doss to provoke a reaction that never comes.

“We both had trouble with that scene,” Gibson says, “because it wasn’t in our nature. Andrew would’ve punched that guy in the face, and so would I.” The director addresses his actor: “Do you remember that? It felt really wrong.”

“It felt raw,” Garfield adds.

“I kept thinking, ‘Is this scene right? Is this going to work?’ ” said Gibson. Still, he adds, when Smitty eventually comes to respect Doss, “You get the sense that, at the end of the day, Desmond won.”

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Another character Doss wins over is Drill Sgt. Howell, played by Vince Vaughn. A frequent USO entertainer with several military members in his family, Vaughn says he worked with Gibson to turn Howell into something more complex than the usual hard-nosed sadist.

“I wanted to create a character that felt unique and authentic and fresh,” says Vaughn. As a drill sergeant, he says, “Your job is really to get these guys prepared not to fold in life-and-death situations. You’re almost a parent to these kids, you don’t want them to die. So I chose to have a lot of love underneath it.”

That echoes Gibson’s somewhat surprising emphasis on the role of love in a film about war. “This is a great message for this troubled time, where we’re in conflict with tribes and people all over the world. And it’s even happening within the U.S.,” Gibson says. “It’s really relevant to try to rise above this horrible broken world we live in and transcend that. As a spiritual way, it’s as good as any and better than most.”