Shia LaBeouf was originally cast, but had to drop out. Charlie Hunnam was then cast, but then was later replaced by Casey Affleck.
Michael B. Jordan was originally cast in the role of Marcus Belmont, but he dropped out due to scheduling conflicts.
The police code "999" that the film is named after does not exist. In real life, Atlanta police use "Signal 63" to indicate an officer down/needs help.