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"It will be rated R," Phillips wrote - referring to the rating that means cinemagoers under 17 would have to be accompanied by an adult.
He added: "I've been asked this a lot. Just assumed people knew. Sorry."
Many people were excited to hear about the 'restricted' rating as it proved we wouldn't be getting a tame kids' movie, with the fan who asked the original question replying: "Thanks for the reassurance, I feared studio interference would be a problem."
Another remarked that last year's Venom would have worked better had it not been a PG-13.
The long-awaited film tells the origin story of the iconic comic book villain, the Joker, who's played by Hollywood icon Joaquin Phoenix.
In the trailer, fans are introduced to the man behind the Joker, Arthur Fleck - a struggling comedian who makes his money dressed as a clown, holding up signs on the side-walk.
But unlike many other depictions of the clown, this follows his descent from doting son to criminal, as we seem him beaten by thugs before saying: "Is it me or are things getting crazier out there?"
Bit by bit, Fleck seems to be losing control. The final shot is of the Fleck, now in full Joker mode, stepping into an elevator and giving a menacing grin as the doors slide shut.
While firm plot details are still fairly thin on the ground, Phillips has revealed what genre the movie will fit into.
"It's a tragedy," Phillips revealed at Cinema Con, where the trailer was given its first showing.
He refused to give much more away, but did admit that a lot of the online discussion surrounding the movie has been incorrect.
"Most of it hasn't been very accurate," he said.
"I guess that's what happens when you set out to do an origin story about a character that doesn't have a definitive origin."
As well as Joaquin Phoenix, the movie will also star Robert De Niro, who plays a talk show host, and Deadpool 2's Zazie Beetz, who steps into the shoes of the Joker's love interest.
Joker is set to hit cinemas on 4 October.
Featured Image Credit: Todd Phillips/Instagram
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