Frank Sinatra loathed the non-singing Marlon Brando for getting the starring role, while Sinatra got a lesser part. His nickname for the sometimes barely comprehensible Brando was "Mumbles."
After filming repeated takes of the scene where Sky (Marlon Brando) and Nathan (Frank Sinatra) first meet, they had to quit for the day when Sinatra had eaten too much cheesecake. He said he could not take one more bite. Brando, knowing how much Sinatra hated cheesecake, had purposely flubbed each take so that Sinatra would have to eat piece after piece of cheesecake. The next day, they came back and shot the scene perfectly on the first take.
Marlon Brando had been cast in the role of Sky Masterson, a role coveted by Frank Sinatra, while Sinatra was relegated to the supporting role of Nathan Detroit. Relations between the two actors were strained during production. Many years later, Brando said of Sinatra, "Frank's the kind of guy who, when he gets to Heaven, is going to give God a hard time for making him bald."
Very few contractions--"aren't" for "are not" or "wouldn't" for "would not," for example--are used in the dialogue in this movie (the songs are a different story). While it makes the language seem stilted and excessively formal at times, this is true to the writings of Damon Runyon. He also eschewed the use of contractions, and this characteristic gave his works a very recognizable style. The reason for this is to emphasize the characters' lack of education and their feeble attempts to appear more learned than they really were.
The decision to cast Marlon Brando was hotly contested, largely by Frank Sinatra, who wanted the part of Sky Masterson himself. Later in his career, he made Sky's big number, "Luck Be A Lady," part of his stage act.