Trivia: The topless girl at the mobsters' pool party is future "Three's Company" TV star Suzanne Somers.
Trivia: Carl Weathers has an uncredited appearance in the film as one of the demonstrators.
Trivia: Clint Eastwood was initially asked to direct, but declined. Ted Post, who had previously directed Eastwood in Hang 'Em High, stepped into the director's seat. However, interviews with Eastwood have indicated that he and second unit director Buddy Van Horn actually directed more of this movie than Post did, due to a number of creative differences. Eastwood and Van Horn would go on to direct the final two entries in the Dirty Harry series, Sudden Impact and The Dead Pool, respectively.
Trivia: After Lou Guzman is killed by a police officer, we cut to a scene where Charlie McCoy is confronted by the officer who shoots him dead in the process. In this scene McCoy was played by another actor as Mitchell Ryan was too ill to film his death scene.
Answer: Because Davis and Sweet demonstrated keen interest, skill and knowledge of firearms (particularly of Harry's .44 Magnum) when they first met. They were flattering Harry, probably in anticipation of inviting him to join their death squad. Harry was impressed and asked for them specifically, even though they were rookies, but he didn't suspect them of being the vigilante killers at that point.
Charles Austin Miller
Yes, he did suspect them. In fact, he suspected Davis from the moment he found out that Davis was the one who reported Charlie McCoy's death. He confirmed his suspicion about Davis after he compared similarities between bullet that killed one of the victims and the one he purposely misfired from Davis' revolver during shooting competition and retrieved it afterwards. He explained it all to Early Smith right after raid on Palancio (and thus unwittingly sealed his partner's fate).