New film “Joker: Folie a Deux" director Todd Phillips, left, and lead actor Joaquin Phoenix speak during a video press conference for the film, Thursday. Courtesy of Warner Bros. Korea

When "Joker" came out in 2019, the film surprised audiences with its new and dark take on the famous supervillain character, exploring the opposite end of the good-fights-evil superhero narrative.

Returning after five years, "Joker: Folie à Deux" comes as a sequel with another surprise. It's a musical.

The new thriller, set for Korean release on Tuesday, picks up two years after failed comedian Arthur Fleck (Joaquin Phoenix) was apprehended for a series of murders, having transformed into the criminal mastermind known as the Joker. Placed in a state hospital while awaiting trial, he discovers the love of his life, Harleen "Lee" Quinzel (Lady Gaga), better known as Harley Quinn, as he grapples with his dual identity as Arthur and the Joker.

Director Todd Phillips noted that it was natural for the sequel to incorporate strong musical elements, as much of the dialogue between Arthur and Lee, as well as Arthur's fantasies, is presented through song.

"In the first movie, one thing (Phoenix and I) spoke about a lot was that even though Arthur is left-footed and out of step with the world, to us, there was a romance in him, and there was music inside of Arthur in the first film. That comes out in the first movie where he's dancing in the bathroom, or he's coming down the stairs in the Bronx," the director said during a video press conference for the film on Thursday.

"So we thought, if we do another one and if he finally for the first time finds love in his life, maybe that music inside of him comes out. So it didn't seem like that much of a leap to us to turn this to have musical elements or for it to become a musical. It felt like an extension of Arthur from the first film and the idea that the music was in him."

The film “Joker: Folie a Deux" will hit Korean theaters on Tuesday. Courtesy of Warner Bros. Korea

He added that he wanted to use the songs to portray Arthur's traumatic past and his relationship with the new light of his life, Lee.

"We wanted it to feel at least Arthur's songs. We wanted to feel that there were maybe songs that his mother played around the apartment when he was growing up," the director said.

"But if you notice, a lot of Arthur's songs are about love, and all of Lee's songs are about power. So they communicate in different ways using music. That was basically how we chose (the songs)."

Phoenix put on quite a musical performance throughout the film, from dancing and singing solo, as well as duets with pop star Gaga.

The actor said despite the vocal training he had to prepare for the film, he intentionally made sure his singing was unsophisticated as it was important to show Arthur's state of mind with the songs.

"First, we had these references, Frank Sinatra and Samuel Davis Jr. And initially I went in, and I tried to perform the songs like the references. Then we looked at each other and I'm like, 'How is that Arthur? He couldn't possibly perform the songs like that,'" Phoenix said.

"It shouldn't be smooth and competent. It should be broken, desperate, bleeding and have cracks. When we figured that out, it gave us a direction to go, 'OK, this is now the voice we've found what his voice is.'"

A scene from the film “Joker: Folie a Deux" / Courtesy of Warner Bros. Korea

He shared that it was a grueling process to practice for musical performances while preparing for the character's gaunt look to reflect his two years of incarceration.

"It was important that he looks worse in the opening scene than we last saw him. So I got lower in weight than in the first film … I actually was dancing two hours a day for like 6 or 8 weeks. And it was difficult to keep my energy up and lose weight," he said, adding Gaga helped him through tough music scenes on the set.

"We were shooting the sequence where I tapped danced, and Stefani (Gaga) was just like banging away on the piano until her fingers were bleeding. She just was giving so much energy and I think that inspired me and fueled me."

Phillips shared how content he is with Gaga's portrayal of Harley Quinn, adding that the singer-actor amazed him.

"(Scott Silver, the film's writer, and I) didn't think of Gaga until we were halfway through the script … So at some point, I was like 'wouldn't it be great if we could find an actor who brings music with them?' and obviously she does that multiplied by 100. But the other thing that the character needed to have, and the thing I wasn't sure about was the vulnerability," he said.

"Obviously, we all know she has a voice like nobody else but what I was most amazed by was how close to the surface her vulnerability was and is. And I thought that really helped round out the character as far as chemistry."