Denzel Washington did ride-alongs with local detectives to prepare for his role. He soon learned that some of the cops carried a second pair of shoes in the trunk of their cars while on duty since the shoes they were wearing would occasionally get blood on them at murder scenes.
During pre-production, Elias Koteas traveled to Ohio to meet a man who could speak Syrian-Aramaic to help him with his dialogue. The man was happy to help, but wouldn't utter the curse words in the speech for religious reasons. Therefore, the producers had to find another Syrian-Aramaic speaker who was willing to speak the profane lines for Koteas.
The name John Hobbes is based on two philosophers from the 17th century, Thomas Hobbes and John Locke. Hobbes generally thought that men were evil, and needed the constraints of society to make them better. Locke thought that men are thinking, rational creatures capable of peaceful co-existence. These two themes are explored in the film.
In post-production, the film was processed by using a technique known as ENR, which deepens shadows and lightens highlights, but doesn't alter any primary colors.