It isn't clear what roles these new cast members will have, but they join the previously-attached Chiwetel Ejiofor, Christoph Waltz, Michael B. Jordan and Casey Affleck. We reported in December that Cate Blanchett was attached, but her involvement hinges on scheduling. It's possible that Kate Winslet may have replaced Cate Blanchett, but that has not been confirmed.
The story centers on a group of corrupt cops who are blackmailed by the Russian mafia to commit an impossible heist. The only way to pull it off is to kill a rookie cop, but their plan backfires when the young officer stops the attack, leading up to a finale filled with plot twists. The title refers to 999, the police code for "officer down."