Sidney Poitier insisted that the movie be filmed in the north because an incident in which he and Harry Belafonte were almost killed by Ku Klux Klansman during a visit to Mississippi. Hence the selection of Sparta, Illinois for the location filming. Nevertheless, the filmmakers and actors did venture briefly into Tennessee for the outdoor scenes at the cotton plantation, because there was no similar cotton plantation in Illinois that could be used. Poitier slept with a gun under his pillow during production in Tennessee. Poitier did receive threats from local racist thugs so the shoot was cut short and production returned to Illinois.
This was the first major Hollywood film in color that was lit with proper consideration for an actor with dark skin. Haskell Wexler recognized that standard lighting used in filming produced too much glare on most black actors and others of dark complexion. Wexler toned down the lighting to feature Poitier with better results.
According to Sidney Poitier, Tibbs' retaliation slap to Endicott was not in the original script nor in the novel on which the film is based. Poitier insisted that Tibbs slap Endicott back and wanted a guarantee that the scene would appear in all prints of the film. According to Stirling Silliphant, the slap was in the original script though not in the novel|