LOS ANGELES − Lady Gaga recalls finding mountains of inspiration for a twisted Joker love story while driving through Big Sky Country in late 2022 with her boyfriend, Michael Polansky.

Polansky, a San Francisco entrepreneur, is not the Joker here, of course. Gaga, 38, doesn't even try to hide the sizable diamond engagement ring from her now-fiancé during an interview. But the Wyoming and Teton Range road trip was awesomely illuminating for her role as Harley Quinn, the infamous love interest to Joaquin Phoenix's DC archvillain in the wildly anticipated sequel to 2019's "Joker."

"So Michael, my fiance, and I were driving around some of the most beautiful mountains in the world," Gaga says. "And I was thinking, 'Can you believe that this woman says to (the Joker) that 'We're gonna build a mountain together'? And I completely believe she believes that. It's a complete fantasy."

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It's a dark illusion at the core of "Joker: Folie à Deux" (in theaters Friday), a warped love story and unconventional supervillain musical starring two of the most electrifying and eccentric actors in Hollywood. Don't go looking for Batman battles or gleeful crime capers from the duo of Joker (mentally disturbed loner Arthur Fleck) and Harley Quinn (fellow mental patient Lee Quinzel).

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No 'Batman.' No bank robberies. So what is 'Joker 2' about?

They didn't fess up to it at the time, but Phoenix and director/producer/co-writer Todd Phillips had long privately discussed a sequel to their R-rated "Joker." This was ensured when the first film went on to earn $1 billion at the worldwide box office, 11 Oscar nominations and two wins − including for best actor for Phoenix, whose Arthur is arrested but celebrated as the Joker at the end of the chaotic origin story. Yet, the follow-up was never going to be a comic-book movie money grab or even what Phoenix, 49, calls "traditional."

"We didn't want to just carry on from the first film like, he's the Joker now, so now we're going to see him out there robbing banks," Phoenix says. "There wasn't going to be any of that."

Phillips and Phoenix started talking about a relationship Joker-style.

"And what happens when somebody falls in love with the image that you project? There was something ripe about that," Phoenix says. "Because even once he's arrested in the original, Arthur is above everything. He's like, there are jokes happening inside me that you're never, ever going to get."

"Folie à Deux" is set two years later, after a TV movie about the Joker's original killing spree, and Arthur is over the mass adoration of his hideously made-up Joker persona. Phoenix got his inspiration by imagining the real daily life of the '70s rock band KISS.

"Did you ever think, how about Gene Simmons from KISS? Where 20-year-olds are painting their faces, putting on platform shoes, all rock 'n' roll," Phoenix says, laughing. "But what happens when you're in your 40s and like, 'I don't want to put the makeup on anymore'? I just started laughing about that. Todd and I were like, maybe that's the beginning of something."

In "Folie à Deux," the daily toll of life in the violent offenders wing of Arkham State Hospital − with daily medications and the oppressive watch of soul-crushing guards − has extinguished Arthur's flame. If Arthur looked alarmingly thin in the original "Joker" as a result of Phoenix's 52-pound weight loss, it's even more apparent in the second movie. Arthur shuffles out of his cell, seemingly more emaciated, all jutting shoulder bones on his exposed back.

"That was the first day of shooting, and I was very, very low," Phoenix says. "I definitely was thinner than I was on the first one."

"That's not CG, that's just his body," Phillips says of the shoulders. "We talked about him not losing the weight again. Because I didn't want him to be unhealthy, quite frankly. But he just thought, 'No, Arthur has to look a certain way.' It was really important to him. If Arthur was a bummer before, now he's a real bummer."

After the success of 'Joker,' Joaquin Phoenix and Lady Gaga found themselves shooting under a 'microscope'

Arthur's despondent perspective changes immediately upon meeting Lee in "Folie à Deux" (a French term that describes a psychiatric syndrome in which two people share the same delusional disorder). The pop icon and actress ("A Star Is Born") had emerged as the clear choice once Phillips and Phoenix decided the film would be a fantasy musical, an idea which had roots in Arthur's famed stair and bathroom dances in "Joker."

Phillips says the downsides of directing two cultural luminaries like Phoenix and Gaga (real name Stefani Joanne Angelina Germanotta) were the swarming paparazzi and fan expectations.

"It was not easy, but not because of their personalities," Phillips says. "When we made the first 'Joker,' nobody knew what the hell we were making. It was all under the radar. Suddenly, especially after you invite Lady Gaga in, you're under a microscope."

Gaga hit the ground belting a Teton-prompted cover of "Gonna Build a Mountain," which appears on her new “Harlequin” album and also inspired an elaborate dream waltz for the budding screen couple that devolves into a nightmare dance.

"It was exciting because I brought the scene idea to Todd and Joaquin," Gaga says. "It starts in the spirit of Fred Astaire and Ginger Rogers. I thought the most wild thing that these two could imagine about themselves is being the picture of health and happiness. But then it falls apart. The waltz is broken, so are they."

The sequel rolls into a sensational, televised Joker trial of the century and Arthur reverts to his wisecracking, garishly face-painted self, urged on by the similarly transformed Harley Quinn. But that's the peak comic-book fare. The singing of standards to the camera and the dancing show the commitment level to the unusual musical.

Obviously, Gaga lives in this world and even had to put the brakes on her voice to make Lee's amateur singing realistic. "Walk the Line" star Phoenix, who took tap lessons at age 8, also has song-and-dance experience. But each large-scale number required enough weeks of training that Phillips feels the need to point out about Phoenix: "There's no CGI, there's no face replacement, there's no stand-in for this dude."

A full-on Joker tap dance interlude required two hours of training each day "for months" with choreographer Michael Arnold, Phoenix says, "and then it's in the movie for like 27 seconds. But, obviously, it's very complex."

During the training, Arnold introduced Phoenix to the expert tap "toe stand" move, balancing on the toe tips.

"I went to do it, and I just stopped," Phoenix says. " I didn't want to break my ankle."

Yet, when he was alone, without pressure, Phoenix found he could pull off the move. Shooting the scene with a belting Gaga on the piano, he felt inspired to let the toe stands rip.

"I think it was just like the energy of seeing Stefani, like the veins were bulging in her neck as she was screaming," Phoenix says. "I was like, 'Well, I've got to try it on film, even if I fall.' So I threw them in. And I couldn't believe I did them. Me and Stefani just got caught up in the spirit."