After Clint Eastwood and Don Siegel came on-board the project, they hired Dean Riesner to work on the script. In his first re-write, the bank robbery scene ends with Harry not pointing the gun at the robber, but placing it against his own temple. He pulls the trigger, laughs, and then walks away. Eastwood and Siegel both felt this was too extreme, even for Harry Callahan.
When Clint Eastwood approached Don Siegel to offer him the directing job, Eastwood gave Siegel four drafts of the script, one of which was written by Terrence Malick. In Malick's script, he had altered Scorpio from being a mindless psychopath killing only because he likes it, to being a vigilante who killed wealthy criminals who had escaped justice. Siegel didn't like Malick's script, but Eastwood did, and Malick's ideas formed the basis for the sequel, Calahan (1973).
After this movie was released, Andrew Robinson received several death threats, and had to get an unlisted phone number.
Serial killer Scorpio was loosely based on the Zodiac killer, who used to taunt Police and media with notes about his crimes, in one of which he threatened to hijack a school bus full of children. The role of Harry Callahan was loosely based on real-life detective David Toschi, who was the chief investigator on the Zodiac case.
The Scorpio Killer (Andrew Robinson ) wears a belt with a peace symbol buckle throughout this movie. According to producer and director Don Siegel, "It reminds us that no matter how vicious a person is, when he looks in the mirror he is still blind to what he truly is."
Don Siegel: Pedestrian walking past Harry's (Clint Eastwood's) car when he and Chico (Reni Santoni ) return to Police Headquarters.