Joaquin Phoenix and Lady Gaga didn’t worry about singing off-key every once in a while. That’s because the “Joker: Folie à Deux” stars were mostly concerned with delivering performances that felt honest and true when it came to pulling off their elaborate musical numbers.

“It was important to me that we never perform the songs as one typically does in a musical,” Phoenix told Variety for a cover story on his “Joker: Folie à Deux” director Todd Phillips. “We didn’t want vibrato and perfect notes.” Instead, the pair “let the emotion guide it” and were mostly in trying to “be true to the moment.”

The film is a sequel to 2019’s “Joker,” which grossed $1 billion at the box office. In the follow-up, Phoenix once again plays Arthur Fleck, an aspiring stand-up comic who is in the psych ward after murdering a talk show host (Robert De Niro) on live television while dressed as a clown. Arthur’s shocking act of violence has inspired a wave of followers, including a fellow patient, Harleen “Lee” Quinzell (Gaga), better known in the comics as Harley Quinn.

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Lee and an army of disaffected Gotham residents believe Arthur’s alter-ego Joker is a prophet of sorts. But things get really crazy when Arthur and Lee find themselves locked in a psychotic duet. But the film isn’t a traditional musical — only Phoenix and Gaga’s characters sing and dance and many of their numbers exist in their addled minds. “Some of the music is fantasy, some of it’s in the scene,” Gaga explains. “It breaks genre.”

Phillips saw the music, which includes covers of such songs as “Get Happy,” “That’s Entertainment” and “For Once in My Life,” as dialogue. “It’s just Arthur not having the words to say what he wants to say, so he sings them instead,” he says.

On set, the collaborators would think about why their characters needed to sing, rather than speak something.

“We asked ourselves what would need to be true for two people to just break into song in the middle of a conversation?” Gaga says. “Where does the music come from when no one can hear it but the characters? Neither Arthur nor Lee are professional singers and they shouldn’t sound like they are (unless perhaps a fantasy). We wanted to help tell the story of their shared madness in a way that felt real. I think we all have an intimate and personal relationship with music in that there’s a score for our inner emotional lives. A score that no one can usually hear but us. That’s what we tried to capture for Arthur and Lee. The music inside them.”

And capturing the “music inside them” required an unorthodox approach. Instead of having Gaga and Phoenix sing along to a pre-recorded track, the actors did everything live, accompanied by a piano player who performed off-camera. In the editing room and throughout the post-production process, these takes were spliced together into one seamless number and the actors later re-recorded parts of the songs. It was an arduous process that Phillips describes as a “nightmare,” but one that was critically important.

“Particularly for Joaquin, so much of it is about feeling the moment as you do it,” Phillips says. “You can’t decide that in a sound studio three weeks before you show up to shoot it.”

Phoenix and Gaga both praise Phillips’ willingness to take risks. “He’s bold and enjoys when something new presents itself,” Phoenix says. “He’s the best at solving problems spontaneously.”

Gaga appreciated his collaborative approach. “As a filmmaker, I don’t think he would ever land in one particular spot and say, ‘This is exactly what I want you to feel.’ I think he just is exploring these two people,” she says.

“Joker: Folie à Deux” will have its world premiere at this fall’s Venice Film Festival before debuting in theaters on Oct. 4.