I believe Warners offered Cary Grant both Music Man in '62 and My Fair Lady in '64. He turned both film roles down, but the famous quote is usually attributed to My Fair Lady, not Music Man. That's where Grant said, "Not only will I not play it, but if you don't cast Rex Harrison, I won't even go see it."
Warners used this same strategy on several adaptations of Broadway musicals: one of the two leads had to be a bankable movie star.
They didn't even care which one. Janis Paige said in several interviews that she knew Warners (her own studio) was considering either Doris Day or Frank Sinatra for the film version of The Pajama Game. When Sinatra turned the part down, she knew she wouldn't get the movie. They cast Doris Day (a big film star), and let John Raitt play his stage role.
With Damn Yankees (also Warners), as long as they had Tab Hunter as Joe, they could use Gwen Verdon as Lola.
With the film versions of Music Man and My Fair Lady, one of the two leads had to be a movie star. Both Robert Preston and Rex Harrison had played leads in films before, but neither one was considered a bona fide bankable movie star. Shirley Jones was fresh from winning an Oscar for Elmer Gantry and had starred in several hit films. And Audrey Hepburn was ... Audrey Hepburn. By casting those leading ladies, it allowed the gents to recreate their legendary stage roles on film.
"Jaws is the Citizen Kane of movies."
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