The SFPD range where Harry meets the four "rookies" is in reality the Oakland (California) Police Department's indoor range. It's now closed, partly due to the many "Dirty Harry" fans who kept trying to sneak into it, but mostly due to indoor inhaled lead problems.
When Clint Eastwood approached Don Siegel to offer him the directing job for Dirty Harry (1971), Eastwood gave Siegel four drafts of the script, one of which was written by Terrence Malick. In Malick's script, he changed the killer from being a mindless psychopath killing because he likes it, to being a vigilante who killed wealthy criminals who'd escaped justice. Siegel didn't like Malick's script, but Eastwood did. Malick's ideas formed the basis for Magnum Force, the second film in the Dirty Harry series.
Directly after the scene in the garage where Callahan is threatened by the motorcycle cops, the cycles drove out and every single one of them crashed. Clint Eastwood said, "I've been threatened by the Keystone Kops."
According to writer John Milius, the reason the sex scene with the Asian woman "Sunny" (Adele Yoshioka) is in the script is because Clint Eastwood received many fan letters from Asian women that contained sexual propositions.
According to Clint Eastwood biographer Marc Eliot the addition of a black partner Early Smith was a "conscious attempt by Warner to ameliorate the outrage that the punk in the 'Do you feel lucky?' scene (in 'Dirty Harry') was black."
The air traffic controller who speaks to Lt. Briggs wears a pocket protector which reads "PATCO". This stands for "Professional Air Traffic Controllers Organization" and was the union whose members were fired by Ronald Reagan for going on strike in 1981.
Clint Eastwood declined the director's seat, and Ted Post stepped in, although interviews with Eastwood and books about him have indicated that he and second unit director Buddy Van Horn actually directed more of the film than Post did.
In the film, Carol McCoy hits on Harry after explaining the circumstances behind her divorce from Charlie. She pulls close to kiss Harry when her rambunctious children's play interrupts them both; a phone call from Earlington Smith to Harry (pertaining to the Cost Plus store stakeout) then forces him to cut short their date and extricates him from a clearly awkward situation. However a publicity photo circulated overseas shows Harry and Carol relaxed and kissing - a different version of this scene where Harry succumbs to Carol's advances was reportedly filmed (and which would have changed the dynamic of Harry's relationship with Charlie) but discarded; it was from this discarded scene that the publicity photo came.
As with the first Dirty Harry (1971) movie, Clint Eastwood performs all his own stunts. His face is clearly visible in several stunt sequences which includes him clinging to the hood of Palancio's moving and swerving car, and the climactic motorcycle chase sequence.
Late in the movie when Harry initially makes his partner, Early Smith aware of his suspicion that a rogue death squad is operating within the department, he make casual reference to a similar situation in Brazil that occurred a few year earlier. This is a reference to the infamous Esquadrão da Morte (death squad) that operated in the mid to late 60s during a time of military dictatorship in Brazil. Led by Detective Mariscot Mariel the squad was made up of rogue members of the police and judiciary, and financed by the business community. Their purpose initially was to assassinate know criminals who could not be brought to justice legally but they later carried out political assassinations and general acts of terrorism against the regime's enemies.
The two escort carriers were ex Rabaul (which was acquired by the navy after WWII but never commissioned and never saw service of any kind) and ex USS Badoeng Strait. (which was completed too late to serve in WWII but saw action in the Pacific - including combat in the Korean War).
The trivia items below may give away important plot points.
Two deleted scenes help explain why Harry grows to suspect John Davis and his pals with the killings of Charlie McCoy and the mobsters. One occurs between the funeral flight for McCoy and the combat championship; after the flight Harry and Davis drive from the airport to a bowling alley for a few drinks; a black youth is suddenly chased outside and assaulted by four toughs; Davis attacks the toughs while Harry dispatches one with his beer mug. After subduing the robbers Davis harangues a group of eyewitnesses for letting such crimes take place; Harry witnesses Davis' harangue and sees in it his own approach to crime fighting, albeit far more severe. Later, after examining the bullet from Davis' gun at the combat championship range, Harry checks on old issues of a police magazine, in which are articles condemning the revolving door justice allowed by liberal politics - articles authored by the four rookie cops. These scenes were deleted presumably because they were judged to be "padding" and not necessary to establishing Harry's suspicion of the four rookie cops.
Actor 'Mitch Ryan' plays a character who gets killed during the film. The day his death scene was scheduled to shoot, Ryan was ill. He brought a doctor's note that read Ryan was "too sick to die" that day.
The motorcycles used by the "death squad" cops through most of the movie are Moto Guzzi Eldorado police bikes. However, Triumph 500cc T100 Tiger motorcycles were used during the scenes on the aircraft carrier at the end. These were favoured by the stunt riders as they were lighter and easier to handle than the Moto Guzzis.
On April 22, 1974, 2 men robbed a HIFI shop in Ogden Utah and made the 5 hostages drink Draino and then shot them in the head. The next day an unnamed informant called in a tip to Ogden City Police with information that would help wrap up the case much sooner than police had anticipated. The informant, an airman stationed at Hill Air Force Base, told police that he had overheard two of his fellow airmen talking about robbing a store and killing witnesses utilizing the Death By Drano method by which the Pimp murders the Prostitute in Magnum Force (1973), which the two had seen prior to the Crime. Two of the hostages miraculously survived. The crime would forever be known as The Hi-Fi Murders.
The plot of the film was inspired by the death squads of Brazil that were in the news at the time. John Milius pitched to Clint Eastwood a scenario of Harry Callahan similarly encountering a corrupt police force of vigilantes assassinating those they could not convict. Eastwood liked the idea particularly since he wanted to address the controversy of the original film supposedly endorsing fascism, by making it clear Harry was not a vigilante.