The role of Harry Callahan was offered to a long list of big stars before Eastwood, including John Wayne, Frank Sinatra, Robert Mitchum, Steve McQueen, and Burt Lancaster.

George C. Scott claimed in a 1980 Playboy interview that he was initially offered the role but turned it down because of the script's violent nature.

Burt Lancaster rejected the lead because he disagreed with the film's violent, end-justifies-the-means morality, while Paul Newman turned it down for being too right-wing and suggested Eastwood instead.

Audie Murphy was initially considered for the villain Scorpio but died in a plane crash before he could respond to the offer; James Caan was also considered when Sinatra was still attached.

Eastwood recommended relatively unknown actor Andy Robinson for Scorpio after seeing him in the play Subject to Fits, and director Don Siegel cast him because he wanted someone 'with a face like a choirboy'.

Andy Robinson is a real-life pacifist who disliked firearms and would flinch every time he had to use a gun, forcing Siegel to briefly halt production to send him for gun-handling training.

Robinson's ad-libbed line 'My, that's a big one,' made the crew break into laughter due to its double entendre, requiring a re-shoot, though the line was kept in the final film.

Clint Eastwood performed the stunt of jumping onto the roof of the hijacked school bus from an overpass himself, without a stunt double, with his face clearly visible.

The script, originally titled Dead Right by Harry Julian Fink and Rita M. Fink, was first written for a New York-set story and went through several drafts, including one version set in Seattle and another by Terrence Malick featuring a vigilante villain also named Davis.

When no lead actor could initially be found, producer Jennings Lang sold the rights to ABC Television for a TV movie, but the violence was deemed too extreme, so the rights were sold on to Warner Bros.

John Milius rewrote the script in three weeks for Sinatra and later joked that his most outrageous ideas, like Scorpio using a flamethrower on the bus and a claymore mine gag, were toned down by Siegel.

The villain Scorpio was loosely inspired by the real-life Zodiac Killer, with additional traits borrowed from kidnapper Gary Steven Krist, who also buried a young girl alive while demanding ransom.

The final scene of Callahan throwing his badge away is considered an homage to a similar scene in the 1952 film High Noon; Eastwood initially resisted the gesture but changed his mind during filming.

Eastwood and Siegel scouted Kezar Stadium as a filming location after watching a San Francisco 49ers game there and being struck by its eerie, Greek amphitheater-like atmosphere.

The film sparked real controversy, including feminist protesters at the 1972 Academy Awards holding banners reading 'Dirty Harry is a Rotten Pig'.

Andy Robinson's portrayal of Scorpio was so convincing that he reportedly received death threats after the film's release and had to get an unlisted phone number.

The film has been linked to several real-life copycat crimes, including the 1972 Faraday School kidnapping in Australia and the 1976 Chowchilla kidnapping, the latter allegedly directly inspired by the film's bus-hijacking scene.

Dirty Harry helped popularize the Smith & Wesson Model 29 revolver chambered in .44 Magnum, leading to a notable increase in sales of the handgun.