John Wayne wanted the role of Harry Callahan in director Don Siegel's 1971 film "Dirty Harry", which was based very loosely on the Zodiac case.

Siegel dismissed Wayne as too old, and Clint Eastwood got the part.

Wayne's last film appearance was in Siegel's 1976 Western "The Shootist", with the screenplay written by Miles Swarthout, the son of Glendon Swarthout, who had written the novel upon which the film was based.

There is a scene in the film in which Wayne, as terminally ill gunfighter J.B. Books comes upon his landlady playing the piano and singing a ditty from Gilbert and Sullivan's opera "The Mikado.". He listens silently until near the end, then pipes up to sing the phrase "tit-willow, tit-willow!"

Zodiac made extensive references to "The Mikado", repeating the "tit-willow" refrain repeatedly in his July 26, 1970 'little list' letter.

I read Glendon Swarthout's novel and found no such scene.

I wrote Miles Swarthout to ask whether he wrote that bit into the screenplay. He replied that he had not, and said "that was all Don Siegel."

Thus, Siegel had Wayne, in effect, quoting Zodiac.