Denzel Washington has frequently said that Detective Alonzo Harris is his favorite character that he's played. He was at first hesitant to take on the role, but one of his sons had read the screenplay and recommended that he accept. Since Washington won an Oscar for the role, he has since always accepted every part that his son advised him to take.
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.
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.
When the movie came out, many viewers and critics were skeptical of the scenes where Jake Hoyt smokes marijuana laced with PCP, and Alonzo's explanation of how a cop who didn't take drugs offered to him on the street would be ID'd as police and murdered. David Ayer responded in an interview by holding up a highlighted section of the L.A.P.D.'s rules and regulations; it stated that officers were allowed to use narcotics in very specific undercover situations, and hewed closely to what Alonzo told Jake.