I don't know if they had a bonafide 'rivalry' that lasted throughout their careers...but it's true that Sinatra (fresh off his Oscar win) was originally cast for the 'On The Watefront' role that eventually went to Brando. And their role 'switch' in 'Guys and Dolls' is a true story too...although I agree if you take away the singing part, Brando was much better suited to the role he played.

Mark Simone of 77-WABC (New York) has a terrific 'Rat Pack' segment on his radio show every Saturday morning. Some obscure story and soundbite from the careers of Frank, Dean, Sammy etc. Last week, he happened to play a
soundbite from sometime during the 1980's, when Sinatra told the story of how he tried to coach Brando during recording of the Guys & Dolls musical numbers. Especially with 'Luck Be a Lady', which Frank said was technically a difficult song to sing. Brando basically thumbed his nose at this attempt of assistance so Sinatra exited the recording studio and left Marlon on his own.

It was a very amusing story the way Frank told it...while making a point of praising Brando as an actor, he made it clear that there was no love lost between the two.

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