“I pitched the idea of an R-rated girl gang film including Harley, because I was like, ‘Harley needs friends.’" Margot Robbie, who will reprise her role as Harley Quinn, said of the film last year. "Harley loves interacting with people, so don’t ever make her do a standalone film. She’s got to be with other people, it should be a girl gang. I wasn’t seeing enough girl gangs on screen, especially in the action space. So that was always a big part of it."

Harley Quinn Spinoff ‘Birds of Prey’ Casts Cassandra Cain (EXCLUSIVE)
By JUSTIN KROLL @krolljvar

Newcomer Ella Jay Basco is in negotiations to land the last major role, Cassandra Cain, in Warner Bros.’ Harley Quinn spinoff, “Birds of Prey.”

Margot Robbie is back as Quinn with Mary Elizabeth Winstead and Jurnee Smollett-Bell on board as Huntress and Black Canary, respectively. Ewan McGregor will play the film’s villain and Rosie Perez will portray Gotham City police detective Renee Montoya.

Cathy Yan is directing the pic with Robbie producing, along with Sue Kroll and Bryan Unkeless.

In the new film, Robbie, Winstead, and Smollett-Bell team up to protect Cain’s character when she comes across a diamond belonging to McGregor’s Black Mask, a kingpin in Gotham City’s criminal underworld.

In DC Comics, Cain is the daughter of assassins David Cain and Lady Shiva, and is deprived of speech and human contact during her childhood as conditioning to become the world’s greatest assassin. She eventually befriends Bruce Wayne and goes on to become Batgirl. It is unknown whether the Batgirl element will be included in this storyline as WB is already developing a “Batgirl” movie.

“Birds of Prey” is set to bow on Feb. 7, 2020.

This marks the first major role for Basco, who has previously appeared in guest stints on TV shows like “Grey’s Anatomy,” “Veep,” and “Superior Donuts.” She is repped by Cunningham Escott Slevin & Doherty.