Phillips' take on the back story of Batman's nemesis has been praised by critics, with Phoenix being tipped to land another Oscar nod for his portrayal of Arthur Fleck.
The movie, which is the first stand-alone film about the legendary comic book villain, follows the story of a failed stand-up comedian, Fleck, who turns to a life of crime and chaos in Gotham City.
Critics have called the film "powerful and original", with Variety calling Phoenix's performance "astonishing".
The actor, who has been nominated for an Oscar three times, said in a recent interview with Total Film magazine he was hesitant to play the villain, previously portrayed by the likes of Jack Nicholson, Heath Ledger and Jared Leto.
"It took me a while (to commit). Now, when I look back, I don't understand why," Phoenix explained. "There was a lot of fear, yeah, but I always say there's motivating fear and debilitating fear. There's the fear where you cannot make a f**king step, and there's the kind where it's like, 'OK, what do we do? That's not good enough'. And you're digging deeper and deeper. I love that kind of fear. It guides us, makes us work harder."
"Joker" hits cinemas in October.