Casting
Paul Dano was cast as the young Wilson and believed, "I don't think it's a totally traditional biopic. I think it's gonna be a fun and accessible film, and I think it's hopefully going to be interesting, like the man that it's depicting. I think he [Moverman] did an amazing job cracking this story, and I can't imagine anyone else having done it, aside from him. I know they tried to make movies about Brian for a long time, and I think music stories are tough to tell in an interesting way." When asked how familiar he was with Wilson before getting involved, Dano answered that he knew the Beach Boys' music, but was not aware of the extent of Wilson's troubles. He proceeded to immerse himself in Wilson's life by reading, listening, and watching as much information pertinent to Wilson as he could, purposefully abstaining from meeting him in person until he was sure he had absorbed everything in full.
Dano reportedly prepared for the role by "filling up" on Wilson's Pet Sounds while learning how to play piano. He explained, "Without question, learning to play and sing and listening to the music was the most important, because the most true Brian to me is in Pet Sounds. If you want to know him, you go listen to it." He related to Wilson's character adding, "Some of it is frankly deeply personal. Brian talked about trying to make music that would heal people. Knowing that his life was so hard and that was his attitude I felt like that's somebody I want to spend time with. Whether I needed love and mercy at the time or I wanted to give it I'm not sure." Of his musical performances, he recalls, "There are a few scenes where you hear me start a line of a song and by the end of the session, you're hearing Brian's vocals. I have to give credit to the sound people, the transitions are really smooth; you can't tell that one half of it is the real thing and one half is me faking it, so I thank those guys a lot." Dano also prepared for the role by gaining weight.
John Cusack, who plays Dano's older counterpart, was chosen for the role after Pohlad watched Don Was' biographical film I Just Wasn't Made for These Times (1995). Pohlad found portraying the 1980s Wilson to be difficult, since he believed Wilson's appearance changed rapidly in that era, but Cusack immediately popped into his head after watching the biography. Pohlad admitted: "He hasn't necessarily always either gotten or taken the greatest roles, but I thought this one would be a good one that would really allow him to shine, and he's truly delivered on that. A lot of people still come up and say, 'Oh, that Paul Dano looks exactly like Brian, but why'd you pick John Cusack? He doesn't look at all like Brian.' Actually, he does, if you look back at that one era." After being selected, Cusack listened to the 2011 release of The Smile Sessions, enthusing, "You get a portrait of the genius at work at the apex of his powers at the time before he kind of went into the ashes," and, "It's hard to overestimate his influence on music. Pet Sounds was a year before Sergeant Pepper's Lonely Hearts, and everything you hear in the Beatles is there. And then you listen more. Especially to The Smile Sessions, and you hear all these other connections. Boom, there's ELO, there's another band." Cusack later recommended I Just Wasn't Made for These Times as a "companion piece" to Love & Mercy.
The decision to have two actors play Wilson was somewhat based on the notion that Wilson today is perceived as a completely different person from when he was in his 20s. Dano believes: "It's important to see the juxtaposition of the two people, especially when you see Brian in the studio so vital, and then see the older Brian quiet, almost traumatized. There is a mystery there, how did that person become that?" Cusack elaborated on his experience with meeting Wilson: "When Brian would talk about the younger version of himself, he was completely dispassionate, like it was someone other than him—in a more extreme way than other people look back on when they were 21. Seeing him talking about it, he just had no ownership of himself." Pohlad encouraged Cusack and Dano not to work together and conjure an impression of Wilson between themselves, and instead try to get an independent sense of who Wilson is from the inside.
Paul Giamatti said of the film, "I play Dr. Landy, the crazy psychotherapist. It's a great character. Brian Wilson had a severe freakout and his family got in touch with a psychotherapist out in L.A. named Eugene Landy who took over. That's where most of the story comes from, because the doctor was basically insane. He made Brian play in a sandbox, I mean crazy stuff." Giamatti prepared for the role by engrossing himself in contemporary articles, meeting the real life Landy's early career acquaintances, and listening to hours of tapes wherein Giamatti says "he'd just keep talking and talking, in these completely huge paragraphs. They were brilliant-sounding, but if you dug into them they didn't make much sense."
