Rooney and Joaquin

“I knew that I could never, ever just ‘act’ in front of Ari — it’s something that I find repulsive and difficult to do,” Phoenix said, calling from New York, where he’s shooting “Joker: Folie à Deux,” the sequel to his 2019 blockbuster. “You couldn’t do that with Ari. He would sense it immediately. It’s nice to know that you are working with somebody like that.”

Phoenix speaks in halting sentences, protective of a process that he’d like to keep in the dark. “I enjoy things that are, at times, difficult to achieve,” he said.

But was Aster funny? “We certainly laughed a great deal,” the actor replied. “What do they call it? Gallows humor. The characters are going through such trauma, you haven’t a choice but to find something funny about it.”

Beyond praising Aster’s standards, Phoenix articulated a bond rooted in risk. “Sometimes it’s as simple as: I could be around you for four months because I like the way you talk about these characters, and I can see that you have a willingness to push yourself, and to be pushed and to push back, and that’s exciting to me,” the actor said.

Aster confirmed that “Joaquin is ruthlessly investigative.” The director said, “If something feels false or not right, he won’t be able to do it. It’s not even about him refusing to do something — his body will stop him.”

Over months of preliminary chats with Phoenix (“a real courtship,” Aster said) and weeks of page-by-page screenplay analysis that extended through more than 60 days of shooting in Montreal during the summer of 2021, a sharper, stranger movie came into focus.

“It was the best experience of my life,” Aster said. “But it also taught me how I want to work in the future — the seriousness with which I expect an actor to approach any given part.”

Teary-eyed and pitched on the edge of panic, Beau is unlike anything Phoenix has created, save for one notorious turn that now feels like a watershed: “I’m Still Here” (2010), supposedly a chronicle of the actor’s breakdown. Aster identified that mockumentary meltdown, poised between humor and cringey self-ruination, as the Phoenix performance that grabbed him.

“What he was doing with his own name was very brave, and arguably suicidal,” the filmmaker said. “And that to me was thrilling.” In coy half-nods, Aster let on that they have another project in development; his next film will “almost certainly” be a western.