VENICE LIDO, Italy — “Joker: Folie à Deux” world premiered Wednesday at the Venice Film Festival where five years ago, “Joker” took home the Golden Lion, the fest’s top prize.

A buoyant Joaquin Phoenix, looking happy and fit in a white tee, co-writer and director Todd Phillips and Lady Gaga discussed why this is no ordinary sequel: their characters Arthur Fleck aka Joker and sociopath Harley Quinn frequently break into song.

“It was something Joaquin and I talked about,” Phillips explained at the press conference.  “Arthur has music in him. Music is a way of expressing what he’s feeling.

“The song selection was an unexpected pleasure, an opportunity to sing with each other. It’s meant to feel like music Arthur would have listened to with his mom and felt like a throwback.”

“Part of the joy for me is,” Phoenix said, “to have songs that were standards — and find a way they were specific to the characters and the only way the characters could express themselves in a way.”

In “Joker,” “I had Frank Sinatra and Sammy Davis Jr, these legends, and we started trying to emulate that sound. Wait a second! This is not who Arthur is — and that’s a point where for me it clicked. We took standards and did our own understanding of them.”

Normally actors record the songs first, then lip sync when filming. Which is what Gaga expected.

“She was, early on, ‘We’re not going to sing live.’ And ultimately we did. And,” Phoenix said, “it was the only way. Not only that we sing live but every part of the recording was live: We were working with a pianist on set and each take was a different version of the song. That energy was necessary.”

“Call me Stefani,” Gaga began, referring to her birth name. “The way we approached music in this film was enhanced. Music is used to let the character express what they want to say.  We allowed the song to come completely out of the character.”

Recently Phoenix had a dramatic walk-off from a new, gay, sexually explicit Todd Haynes movie he was to star in. His quitting prompted an immediate shutdown and an estimated million-dollar loss.  Ironically, the picture was Phoenix’s idea.

Despite speculation in Hollywood that he’s destroyed his career, he’s offered no explanation, said nothing.

Until he was asked to comment at the press conference and said, “I think if I do I will just be sharing my perspective — and the other creators aren’t here to share and I don’t think that would be helpful. So I don’t think I will.”

“Joker: Folie à Deux ”opens nationwide theaters only on Oct. 4

(Photo Warner Bros.)