Acting in the Dark: During the scene when Jake plays cards with the Latino gangsters, director Antoine Fuqua gave certain instructions to the actors playing the gangsters, without telling Ethan Hawke. This was done to further confuse him and add to the tension of the scene.
Saved from Development Hell: David Ayer wrote the first draft in 1995, way before the Rampart Scandal made headlines in 1998. In the first draft, Alonzo was a Latino officer in charge of drug enforcement in the gang infested area of Pico-Union within the Rampart Division. He was a loner and didn't have any partner because of his reputation as a corrupt and racist cop. But the basics elements of the character were there: he wears expensive jewelry and drives a 1979 Monte Carlo.
Before Antoine Fuqua was involved in the project, Davis Guggenheim was set to direct with Samuel L. Jackson as Detective-Sergeant Alonzo Harris and Matt Damon as rookie Officer Jake Hoyt. When Denzel Washington accepted the role as Detective-Sergeant Alonzo Harris, Guggenheim was replaced per his request.
Tobey Maguire was seriously considered for the part of Officer Jake Hoyt. He even followed Undercover Narcotics officers in Los Angeles for two months and gained weight for the role. He was dropped when Ethan Hawke, Fuqua's first choice, was finally available for the shooting schedule.
Antoine Fuqua wanted Mickey Rourke to play Roger, but the studio turned him down.
The original ending of the script was Hoyt crashing the pad of Alonzo's girlfriend and find his training officer in the middle of an orgy with two young women. Hoyt shows him the money that Smiley was supposed to deliver to the Russians and tells him that there is a crew of hit men looking all over for him. Alonzo then commits suicide in front of Hoyt.
Author Existence Failure: Bill Paxton passed away on February 2017 due to complications from surgery. The production team has since stated that another actor will not be casted as Frank Roarke (though due to the show's already flagging ratings, Paxton's death was likely just the final nail in the show's coffin).