In an interview with the Los Angeles Times, director Todd Phillips spoke about the film's production, and how it took a while to get people on board with the ideas he had in mind.
He said: "It was a year-long process from when we finished the script just to get the new people on board with this vision, because I pitched it to an entirely different team than made it."
In that time, Phillips also claims Phoenix lost 52 pounds in order to play the iconic Batman villain.
He continued: "We wanted to look at everything through as real and authentic a lens as possible.
"I don't believe that in the real world if you fell into a vat of acid you would turn white and have a smile and your hair would be green. So you start backwards-engineering these things and it becomes really interesting. 'How about if he's a clown at one of these places where you rent out entertainment?'
"It was one of the most fun scripts to write because you were only breaking rules."
In the interview, Phoenix added: "That's really the only thing that's worthwhile; the other thing is connect-the-dots and paint-by-numbers, and who the [heck] cares about that?"
He continued: "There are certain areas of the character that frankly still aren't clear to me, and I'm fine with that. There's something enjoyable about not having to answer a lot of those questions. It requires a certain amount of participation from the audience that feels different."
The film will follow failed stand-up comedian Arthur Fleck as he turns to a life of psychopathic crime after society rejects him - in turn transforming himself into the Joker to wreak havoc on Gotham City.
As well as Joaquin Phoenix and Bobby De Niro, the film also stars Deadpool 2's Zazie Beetz as Arthur's love interest.
In a recent interview with Empire, Phillips also explained how Joaquin Phoenix had always been the man for the role.
"I think he's the greatest actor," he said.
"We had a photo of him above our computer while we were writing. We constantly thought, 'God, imagine if Joaquin actually does this'."
Phillips continued: "We just wrote our own version of where a guy like Joker might come from. That's what was interesting to me. We're not even doing Joker, but the story of becoming Joker. It's about this man.
"We didn't follow anything from the comic books, which people are gonna be mad about."
Joker is set to hit cinemas on 4 October.