In all of his years of acting, stretching back to when he was 8 years old, Joaquin Phoenix has never done a sequel. And heading into “Joker,” he didn’t expect that to change.

A grim, gritty take on the origin of arguably the comic-book world’s most iconic villain, director Todd Phillips’ film was designed as a standalone story that would exist — in every conceivable way — outside of the DC cinematic universe. That was a major part of the appeal to Phoenix, who had turned down earlier offers to star in comic-book movies out of a concern that he’d find himself sucked into the maw of a commercial machine.

“I guess the fear was that you’d get locked into doing something repeatedly that you don’t really care about, that doesn’t motivate you or excite you,” Phoenix said during an interview for an upcoming cover story in The Envelope. “Part of the whole attraction to me [of ‘Joker’] was there was no expectation. I didn’t sign a deal to do [more movies]. It was a one-off.”

Now that the pre-release controversy has abated and the film has become a box office smash — earning some $850 million worldwide to date and setting a record for the highest-grossing R-rated movie of all time — those gears of commerce are surely turning in the minds of Warner Bros. executives. Still, Phoenix — whose turn as troubled would-be comedian turned murderous supervillain Arthur Fleck has landed him at the heart of this year’s best actor Oscar race — insists he and Phillips would never do a follow-up simply because Hollywood logic demands it.

“I wouldn’t just do a sequel just because the first movie is successful,” he said. “That’s ridiculous.”

That’s not to say Phoenix is completely opposed to a sequel, though. While they were making “Joker,” Phoenix says he and Phillips, previously best known for directing hit comedies like the “Hangover” trilogy, were mulling over the idea of further plumbing Fleck’s complex psyche.

“Long before the release or before we had any idea if it would be successful, we talked about sequels,” Phoenix said. “In the second or third week of shooting, I was like, ‘Todd, can you start working on a sequel? There’s way too much to explore.’ It was kind of in jest — but not really.”

In fact, at one point during filming Phoenix had posters mocked up with the Joker inserted into various old movies as a tongue-in-cheek way of showing Phillips what might be possible.

“I basically said, ‘You could take this character and put him in any movie,’ ” Phoenix said. “So I did a photo shoot with the on-set photographer and we made posters where I Photoshopped Joker into 10 classic movies: ‘Rosemary’s Baby,’ ‘Raging Bull,’ ‘Yentl …’ ” He laughs. “If you see it, you’re like, ‘Yeah, I’d watch that movie.’ ‘Yentl’ with Joker? That would be … amazing!”