Kirk Douglas was so keen to play Don Quixote that he offered Paramount $1m for the rights. In 1965, Anthony Quinn negotiated with Dale Wasserman for the film rights. Quinn wanted Mexican comedian Cantinflas for the role of "Sancho Panza" (with Quinn as Quixote).
Peter O'Toole recorded his vocal tracks for the film, but after realizing his own voice was not sufficient enough for the requirements of the music, assisted in the search for a singing voice double. The man O'Toole initially picked sounded nothing like him, so a new search was begun, and eventually Simon Gilbert was selected for singing most like O'Toole's speaking voice.
Librettist Dale Wasserman omitted from the screenplay what was apparently one of his most favorite lines from the play, uttered by Don Quixote, "Facts are the enemy of truth."
This was one of the last films to receive a limited-release, reserved-seat "roadshow" engagement prior to its general release. Les horizons perdus (1973) was the last of the "big film musicals" to receive this kind of release during the time period.
Even Sophia Loren queried why the film was shot in grubby soundstages in Rome instead of the glorious locations of Andalucia where the film is supposedly set.