Phillips posted a picture Sunday on Instagram of Phoenix as Arthur Fleck, a failed stand-up comedian who becomes the villain known as the Joker.
The photo shows Phoenix applying white face makeup in front of a mirror. The mirror's angles create two reflections of the actor.
"Finishing touches," Phillips captioned the post.
Phillips confirmed in the comments that Joker will be rated R.
"It will be Rated R. I've been asked this a lot. Just assumed people knew. Sorry," he wrote.
Joker opens in theaters Oct. 4. The film released a first teaser trailer in April showing Arthur's descent into madness.
"It feels unique, it is its own world in some ways, and maybe, mostly, it scares the [expletive] out of me or something," Phoenix said of the movie in an interview with Collider in July 2018.
"It feels unique, and I think more than anything, and probably the most important thing, is Todd seems very passionate about it and very giving, and so that's exciting," he added.