Paul Giamatti, John Cusack and Elizabeth Banks in “Love & Mercy.” (Francois Duhamel/Roadside Attractions)

It’s a little weird talking to two Brian Wilsons at the same time.

Wilson and John Cusack, who plays the former Beach Boy in the new biopic “Love & Mercy,” were supposed to take turns answering questions in a joint phone interview. But the actor ended up doing most of the talking, including delivering answers from time to time on Wilson’s behalf — and with Wilson’s permission — in response to questions that the 72-year-old either couldn’t hear or was simply tired of answering, in what was certainly not his first interview of the day.

For example, regarding the synergy between the film, which was made with the blessing of Wilson and his wife, Melinda, and a forthcoming autobiography, “I Am Brian Wilson,” Cusack jumps in, like a good spokesman: “I think Brian would say that the book and the movie, because Melinda and he were involved in making both of them, is a very accurate representation of his life.”

The musician seems slightly confused at times. But whether that’s because of his deafness in one ear or the result of past drug use — “I burned my brain out,” he told Diane Sawyer in 1991 — isn’t clear. More than anything, Wilson comes across as limelight-averse, even painfully shy. “Brian is not a huge fan of social interaction,” director Bill Pohlad told the New York Times last year, when it wasn’t clear whether the singer, songwriter, producer and musical polymath would even be available for interviews.

Pohlad isn’t even sure whether Wilson realized what he was getting into when he agreed to collaborate with the filmmaker, opening his life up for reexamination. “I don’t think he did, in a lot of ways,” the director says.

Composer and song writer Brian Wilson. (James A. Parcell/The Washington Post)

“Love & Mercy” focuses on two main threads. The first concerns the Beach Boys’ meteoric rise to pop stardom in the 1960s with a string of 11 albums, culminating in 1966’s “Pet Sounds” (a commercial failure now widely acknowledged as one of the greatest albums of all time). The second thread involves Wilson’s well-publicized mental breakdown, drug addiction and subsequent recovery from dependence on the controversial psychotherapist Eugene Landy, who exercised almost total control over Wilson’s medical, personal and financial affairs from 1982 to 1991.

Of Paul Giamatti’s creepily Machiavellian performance as Landy, who died in 2006, Wilson calls it “letter perfect.”

“I almost wanted to walk out of the theater,” Wilson says of his reaction to the Landy scenes. “I was so depressed.” At the same time, he acknowledges that the depiction of his creative peak was a blast. “The good times were good,” he says, with the kind of cryptic understatement that makes Cusack describe him as a “Cheshire cat.”

In an un­or­tho­dox bit of casting, Cusack, 48, plays Wilson in the 1980s and Paul Dano, 30, plays him in the 1960s. As the movie cuts back and forth between those dueling Brians, each of the performances, in its own way, contributes to a prismatic whole.

According to director Pohlad, the double-casting was essential to what he calls a “two-strand” approach to capturing Wilson’s life. “You can’t tell Brian’s story without telling about the ‘Pet Sounds’ era,” he says. “‘Love & Mercy’ is the story of a hypercreative musical genius reaching his peak, really, his most creative period. And then he falls off the edge.”

If Dano’s Wilson is the manic genius teetering on the precipice, Cusack’s performance presents an entirely different persona — one we meet many years later as he’s crawling out of the hole he has fallen into. Cusack’s character seems shell-shocked, almost catatonic, caught as he is in the throes of Landy’s psychological and pharmacological ma­nipu­la­tions. In addition to taking LSD and other illicit drugs, Wilson had been prescribed lithium, Xanax, Halcion and other psychotropic medications, all under the supervision of Landy’s in-house psychiatrist, Sol Samuels.

The first strand of the film follows Wilson’s descent into madness. The second picks up the narrative just as the singer is meeting and falling in love with his future wife (Elizabeth Banks), whom Wilson credits with helping him escape from what he calls Landy’s prison.

In a sense, Pohlad says, it’s almost as if these two Brians aren’t even the same man. “The Brian of today is significantly different from the Brian when he’s 20-something,” says Pohlad, a veteran producer of such prestigious fare as “12 Years a Slave,” “The Tree of Life” and “Brokeback Mountain.”

Initially asked by producers John Wells and Claire Rudnick Polstein to help produce an earlier screenplay based on Wilson’s life called “Heroes and Villains,” Pohlad wound up in the director’s chair almost by accident. “I read that script, but I didn’t like it at all,” he says, calling “Heroes” more of a “traditional biopic-y, melodramatic thing.” Although the idea to tell Wilson’s story by jumping between two eras — and two actors — was Pohlad’s, he hadn’t planned on directing until the writer he had hired, Oren Moverman, suggested he should, in order to protect his vision.

“There were those who said, ‘You know, you should really do this with one actor, because it would be a tour de force for somebody like Ryan Gosling,’ ” Pohlad says with a laugh.

Even riskier than the double-casting was Pohlad’s decision to implement a kind of voluntary quarantine on Cusack and Dano, neither of whom ever consulted with — or even watched — the other during filming.

“I never liked the idea of them going, ‘Why don’t we both move like this? Or why don’t we both scratch our nose like that?’ ” Pohlad says. “John and Paul were both very on board with that idea. There was something intriguing for them about the two-actor thing, I think, that played out in the way they found their Brians.”

Cusack, for instance, chose to meet extensively with Brian and Melinda. “I hung out as much as I could,” he says, “without becoming the guest who wouldn’t leave.” Dano, on the other hand, steered clear, instead poring over archival films of the early Beach Boys and listening to tapes of the “Pet Sounds” recording sessions.

A singer and guitarist for the band Mook, Dano immersed himself in the technical minutiae of those recordings, according to Pohlad, for whom musical verisimilitude was paramount. Period accuracy was ensured by the services of Mark Linett, a producer with a long association with Wilson who came on board as a consultant (and who can be seen in the film’s recording-studio scenes, playing real-life sound engineer Chuck Britz.)

Wilson’s music also was key to helping Cusack unlock his character’s troubled psyche. “John would be listening to the ‘Smile’ sessions right up to the moment we said action,” Pohlad recalls, referring to the follow-up album to “Pet Sounds,” whose abortive and contentious creation precipitated Wilson’s breakdown. “He’d pull off his headset, toss it off to the side and do the scene.”

Both Cusack and Pohlad say they found Wilson’s character by feel, rather than observation. “I don’t think you can play Brian as an intellectual thing,” Cusack says. “You can’t just do a take on him. You have to try and feel what he felt.” The answer, Cusack believes, lies is the man’s complex music. “There’s joy, there’s longing, there’s heartache, there’s triumph.”

Such complexities are echoed in Wilson’s life.

Much has been written about him over the years, including his 1991 autobiography, “Wouldn’t It Be Nice: My Own Story,” now disavowed as having been ghostwritten by Landy. But there are many details that don’t line up. Wilson, for instance, has variously attributed his deafness to a congenital condition and to physical abuse by his father. (The film comes down on the abuse side, because, Pohlad says, that’s what Wilson has always told Melinda.)

Pohlad insists that he isn’t trying to create the definitive — or even an authorized — version of the Brian Wilson story. “This is my portrait of Brian,” he says. “All I can do is be as respectful as possible to my vision of it. Hopefully, along the way, part of that is being responsible to Brian and to his life. But you never know where the truth is. You can’t stray into that area where you’re trying to protect someone, or trying to change the story so they’re super happy with it.”

Happy may be overstating it. But the taciturn former Beach Boy is suddenly effusive when asked what he thinks about “Love & Mercy.” “I thought the film was ingenious, sensitive and loving,” he says, just before hanging up the phone.