"So excited to be working with @zaziebeetz. Joker," Phillips said alongside a photo of Beetz in a sweater, staring off to the side.
Beetz's character in the film has been described as a single mother who catches the eye of Phoenix.
Phillips, who co-wrote the film's script with Scott Silver, recently released on Instagram a video of Phoenix in makeup as Joker, whose real name will be Arthur Fleck.
Joker is set to arrive in theaters on Oct. 4, 2019. Robert De Niro, Marc Maron, Bill Camp, Frances Conroy, Brett Cullen, Glenn Fleshler, Douglas Hodge Josh Pais and Shea Whigham also star.
Joker takes place outside of DC's extended film universe and would help launch a new DC Comics banner at Warner Bros. that will allow the studio to tell unique comic book stories using different actors in the the title roles.
"Joker centers around the iconic arch-nemesis and is an original, standalone story not seen before on the big screen," reads the synopsis. "Phillips' exploration of Arthur Fleck, a man disregarded by society, is not only a gritty character study, but also a broader cautionary tale."