Jared Leto: He’s played a dreamboat and some unsavory characters and been a rock star. Most recently, he packed on 67 pounds to play John Lennon’s killer in ‘Chapter 27.’

ALTHOUGH he began his career as “My So-Called Life’s” adolescent object of desire, Jared Leto’s work since has been marked by a kind of reverse vanity. The uglier the role, the better.

Still, the lengths to which Leto went for his latest role might seem just a tad extreme. To play John Lennon’s assassin, Mark David Chapman, in J.P. Schaefer’s new indie film, “Chapter 27,” opening in limited release Friday, Leto packed 67 pounds onto his lanky frame to better approximate the killer’s doughy physique. By the end of filming, the extra weight had taken such a toll that he could no longer walk to the set.

“I’m not sure it was the wisest choice,” he admits. “A friend of mine was recently going to gain weight for a film and I did my best to talk him out of it. Just because you can lose the weight doesn’t mean the impact it had on you isn’t there anymore.”

While Leto’s made a point of tackling challenging roles in dramas including “Fight Club,” “Requiem for a Dream” and “Lonely Hearts,” he says that “Chapter 27” presented one of the greatest challenges he’s yet faced as an actor. Namely, he had to put his own judgments aside to capture the essence of the character.

“As unfortunate as it can be to hear this, he was a human being,” Leto says. “A failure of a human being. A very sick and disturbed individual. But when you’re an actor playing a part, you don’t play a monster. Or at least I don’t know how to do that. You have to try to understand the human being there.”

In addition to changing his body for the film – the title refers to Chapman’s belief that he was carrying on the ideas laid down in J.D. Salinger’s “The Catcher in the Rye,” which has only 26 chapters – Leto altered his voice to echo Chapman’s eerily courtly Southern drawl. “He was very bottled up and you could hear it in his voice,” Leto says. “Everything was closed off. He barely ever speaks above a whisper and everything is kind of choked off in the throat. He was, in the strangest way, a very polite, genteel kind of person, and he certainly thought of himself as a gentleman, which is so strange for someone committing such a horrifying and despicable act.”

Leto relied on interviews with Chapman, who is serving 20 years to life at Attica prison, for reference, and on audiotapes recorded by an elderly librarian the actor met during a visit to Chapman’s hometown of Decatur, Ga. “I pored over that for hours and hours,” Leto says. “You can’t find someone in this day and age who talks like this. It’s kind of from another time.”

Next for Leto is Jaco van Dormael’s “Mr. Nobody,” a time-travel odyssey in which Leto plays 12 versions of the same character, “including a 120-year-old man and a Neanderthal.” Among those personae is a role that even Leto finds intimidating: a normal guy.

“I wasn’t playing a drug addict or a psychopath, but one of the characters is married with kids,” he says. “Playing a father was one of the more terrifying things that I’ve had to do on film.”