Scorpio was loosely based on the real-life Zodiac Killer, an unidentified serial killer who had committed five murders in the San Francisco Bay Area several years earlier. Elements of Gary Stephen Krist were also worked into the characterization, as Scorpio, like Krist, kidnaps a young girl and buries her alive while demanding ransom. In a later novelization of the film, Scorpio was referred to as "Charles Davis", a former mental patient from Springfield, Massachusetts who murdered his grandparents as a teenager. There are significant differences between the book and the film. Among the differences are: Scorpio's point of view — in the book he uses astrology to make decisions (including being inspired to abduct Ann Mary Deacon); Harry working on a murder case involving a mugger before he is assigned to Scorpio; the omission of the suicide jumper; and Harry throwing away his badge at the end. Audie Murphy was initially considered to play Scorpio, but he died in a plane crash before his decision on the offer could be made. When Kershner and Sinatra were still attached to the project, James Caan was under consideration for the role of Scorpio. The part eventually went to a relatively unknown actor, Andy Robinson. Eastwood had seen Robinson in a play called Subject to Fits and recommended him for the role of Scorpio; his unkempt appearance fit the bill for a psychologically unbalanced hippie. Siegel told Robinson that he cast him in the role of the Scorpio killer because he wanted someone "with a face like a choirboy". Robinson's portrayal was so memorable that after the film was released he was reported to have received several death threats and was forced to get an unlisted telephone number. In real life, Robinson is a pacifist who deplores the use of firearms. Early in principal photography on the film, Robinson would reportedly flinch in discomfort every time he was required to use a gun. As a result, Siegel was forced to halt production briefly and sent Robinson for brief training in order to learn how to fire a gun convincingly.