Phoenix is 44, with hair that is a mixture of brown, copper and gray strands, and he spoke with an unexpected gentleness, like Commodus, the wicked emperor he played in “Gladiator.” (We know how it turned out for Commodus.)

Phoenix could be playful at times. When I noted how nimble he looked in some of his dancing scenes in “Joker,” he swatted away the compliment, saying, “I would get injured just from doing a light jog down the street. I’d have to be sent home.”

But some of that lightness evaporated as soon as I asked how he’d been approached about the film and he replied that he could not remember. “It sucks — this is why interviews are the worst,” he said despairingly, adding that he was tempted to make up a story “just to sound exciting.”

Nor was he in any hurry to explain his process for figuring out his “Joker” character before filming began. “It’s so stupid to talk about,” he grumbled. “I’m not going to talk about it.” (He did eventually talk about it.)

LET’S SET ASIDE PHOENIX for the moment and return to Phillips, who is best known for directing the lucrative “Hangover” comedies. At the premiere of his 2016 crime caper, “War Dogs,” Phillips found himself anticipating its tepid reception while gazing at a billboard for a Marvel superhero juggernaut. He wondered how he could possibly compete.

Warner Bros. had been having only intermittent success with its DC superhero movies — “Wonder Woman,” yes, “Suicide Squad,” no — but Phillips saw a potential solution to everyone’s problems. “You can’t beat Marvel — it’s a giant behemoth,” he said. “Let’s do something they can’t do.”