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.
The now classic scene between Burt Lancaster and Deborah Kerr in the rushing water on the beach was not written to take place there. The idea to film with the waves hitting them was a last-minute inspiration from director Fred Zinnemann.
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.