Casting
One is of course well aware of how often a stage star was dumped for the film version of a musical....I am aware of only one major musical that used all of its starring Broadway cast and that is 1776. (some of the smaller parts were recast,notably Rutledge but the top stars were the same.) At best when we were lucky at least one star of the musical made it to the film. Usually it was a case of the male lead being so perfect in the role that it was considered impossible to have any other actor in the role. 2 such cases are The King and I and My Fair Lady. Now it didnt really matter about recasting Anna in the the King and I because Gertrude Lawrence had died anyway, but as everyone knows it became a major issue with the My Fair Lady, as Jack Warner insisted that if Rex Harrison played Higgins he had to have a star as Eliza and substituted Audrey Hepburn for Julie Andrews. Audrey had to be dubbed since although she could sing she couldnt sing songs written for Julie Andrews. What all this is leading up to is that you had a similar situation with The Music Man....In order to have Robert Preston, there had to be a star actess as Marian to ensure the box office...and the movie would never have happened at all if Preston hadnt been cast, so lets all be grateful for that. But fans of the Music Man got luckier than other viewers because they cast someone who could sing and act in the role. I think that of all the times a movie star was subbed for an original Broadway star this was the luckiest case. Shirley Jones is absolutely right in the part.
It is not our abilities that show who we truly are...it is our choices