The actor previously played the role in 2016’s critically panned supervillain team-up film Suicide Squad, but did not appear in the original Justice League.
Snyder’s re-editing of Justice League is being undertaken for the streaming service HBO Max, and came at the behest of fans who spent the years since Justice League’s original debut demanding the release of “the Snyder cut”.
The project will be released on HBO Max next year, reportedly in the form of a four-episode limited series.
The news of Leto’s casting has not been met with unanimous approval from fans, however.
Leto’s preparation for the role and method acting techniques were controversial at the time, with the actor infamously sending his co-stars used condoms in the post.
His performance as Joker, following on from Heath Ledger’s posthumous Oscar-winning portrayal in 2008’s The Dark Knight, was also met with mixed reviews.
“I truly dont understand the people that like his Joker,” wrote one commenter on Twitter.
“Joaquin Phoenix was so impactful as Joker [in 2019’s Joker]. It’s one of those situations where anybody else who takes on the role will fall short,” wrote someone else.
“So to go back to someone who was arguably the worst Joker (not his fault) is a terrible idea.”
“Is this whole ‘desperately want the Snyder Justice League cut released’ an internet joke that got out of hand?” asked another. “I thought it was genuine, but now they're pretending genuine enthusiasm about the return of Leto's Joker? That can't be real, can it?”
Others, however, were more forgiving, with many fans either arguing in favour of his Suicide Squad interpretation of the character, or saying that he deserved to be given a second chance to shine.
“Honestly, i feel like he deserves another shot considering Suicide Squad as a whole wasn’t that great,” wrote one Twitter user.
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