The first-look comes a day after Warner Bros. released a camera testing showing Phoenix in his Joker-esque clown makeup.
Beetz plays hardened single mother Sophie Dumond, who is "worn out by the grind of the city" and "just trying to catch a break" in life in the Lower East Side. Dumond has been touted as a love interest for Phoenix's proto-Joker, Arthur Fleck.
The actress recently told E! News she is "just such a huge fan" of her co-star Phoenix, saying "it's just such an honor to work with him. And to follow him and his work, I think, it's like anyone else. I'm human."
Pressed for details on her role, Beetz said she "can't" reveal anything. "I'll probably lose my job if I [say anything]."
Beetz broke out in Donald Glover's Atlanta FX series before landing a major role in Fox Marvel Comics adaptation Deadpool 2 this summer, where she played fortuitous mutant Domino.
Phoenix attributed his stepping into the comic book genre to Phillips, who Phoenix says is "very impressive."
"I take a lot of time and consideration when making decisions and what I'm gonna work on always," the three-time Oscar nominee told Collider.
"So, in some ways, the process, which is obviously reading a script and meeting a filmmaker and then continuing to have meetings and discussions with Todd [Phillips]. I think he's very impressive and he seems to have a very interesting understanding of this world and what he's trying to say."
Phoenix said he found "something very appealing about that and working with him on this particular project," adding it "feels unique" and is "its own world in some ways, and maybe, mostly, it scares the f—ing s—t out of me or something. It might as well be the thing that scares you the most."
Joker is now shooting for an October 4, 2019 release.