Todd Phillips says comic book fans may not be pleased with the way his Joker has shaped up as the new film is the director's own version of the DC supervillain's story.
Phillips, who also co-wrote the screenplay with Scott Silver, said the plot is more about how Joker becomes the man he came to be known as.
"We didn't follow anything from the comic books, which people are gonna be mad about.
"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," he told Empire magazine.
It was earlier speculated that the movie might follow portions of Alan Moore and Brian Bolland's legendary The Killing Joke, the comic that provided the origin story for the Clown Prince of Crime.
The book told of an aspiring comedian whose mob dealings eventually land him into a vat of chemicals at the hands of Batman.
Hailing Joaquin Phoenix as "the greatest actor", Phillips also talked about collaborating with the film's lead.
"We had a photo of him above our computer while we were writing. We constantly thought, 'God, imagine if Joaquin actually does this'," the director said.
"Joker", rated 'R', is slated to be released on October 4.