Montgomery Clift threw himself into the character of Prewitt, learning to play the bugle (even though he knew he'd be dubbed) and taking boxing lessons. Fred Zinnemann said, "Clift forced the other actors to be much better than they really were. That's the only way I can put it. He got performances from the other actors, he got reactions from the other actors that were totally genuine."
In the scene where Burt Lancaster and Montgomery Clift play drunk sitting on the street, Clift actually was drunk, but Lancaster was not.
Frank Sinatra had personal problems of his own. The collapse of his marriage to Ava Gardner weighed heavily on him; it got so bad he announced to Montgomery Clift one night that he was going to kill himself.
An urban myth regarding the casting of Frank Sinatra was that the Mafia made Columbia Pictures an offer they couldn't refuse. This, of course, was fictionalized in Mario Puzo's novel The Godfather (1972) and its subsequent film adaptation. The real reason for Sinatra's casting was mainly his then-wife Ava Gardner, who was shooting a film for Columbia head Harry Cohn and suggested to him that he use Sinatra. Although initially reluctant, Cohn eventually saw this as being a good idea, as Sinatra's stock was so low at the time that he would sign for a very low salary. Sinatra had been lobbying hard for the role, even suggesting he would do it for nothing, but he was eventually hired for the token amount of $8,000.
Montgomery Clift, Frank Sinatra and author James Jones were very close during the filming, frequently embarking on monumental drinking binges. Clift coached Sinatra on how to play Maggio during their more sober moments, for which Sinatra was eternally grateful.