Armie Hammer owes Woody Allen.

To prepare for the title role in “Billy: The Early Years,” Hammer watched every Billy Graham sermon he could get his hands on, faithfully studying each gesture and Southern-fried inflection. But it wasn’t until the Los Angeles-born actor stumbled across a lively YouTube clip of the director interviewing the evangelist on a 1969 TV special that he really saw the light.

“You look at it and go, ‘Man, Billy Graham really has a sense of humor,’ ” Hammer says. “He’s holding his own with Woody Allen and cracking the audience up. That was the moment when I felt I could see the man behind the preacher.”

It’s precisely that human side of the venerated icon that the biopic earnestly seeks to capture. And with his all-American good looks and robust enthusiasm, Hammer — cast after another actor bowed out due to a scheduling conflict — nimbly nails Graham’s transition from a religion-leery aspiring baseball player to world-renowned spiritual leader.

The 6-foot-5 newcomer claims he wasn’t intimidated by the prospect of portraying such a towering figure, thanks to actor-turned-helmer Robby Benson.

“Robby’s so quiet and humble,” Hammer notes of the director, “but he’s willing to teach you everything he knows as long as you’re willing to listen. I knew I was in good hands.”

It was Benson who suggested Hammer have dinner with Oscar winner Martin Landau, who plays Graham’s one-time mentor, Charles Templeton. (Because of the flashback nature of the film, the two never share any scenes.)

“Mr. Landau said, ‘Do you mind if I say something?'” remembers Hammer, who is the great-grandson of oil tycoon Armand Hammer. “He didn’t stop talking for three hours, but at no point was I going, ‘Oh, this old guy’s rambling.’ I was enraptured by every single word. When a legend says anything, you’re like, ‘That’s the most important thing I’ve ever heard.’ ”

Hammer had planned to follow up “Billy” by playing Batman in Warner Bros.’ “Justice League,” but plans for the film have since been delayed. Instead, he’s headed to Vancouver to shoot a five-episode guest stint on the CW’s “Reaper,” where he plays the devil’s son.

His move from the heavenly to the hellish isn’t lost on him. Says Hammer with a laugh, “Hey, at least I’m not getting typecast, right?”

An actor should always:
“Bathe. You’re not gonna get a job smelling to high heaven.”
Lucky break: “‘Justice League’ got all the wheels turning. Whether that (project) happens now or not, I’ve already experienced the benefits of being cast.”
Favorite film character: Gary Oldman’s crooked DEA officer in “The Professional.” “That’s one of the finest bits of acting I’ve ever seen. He fully embraces the darkness.”