The author of the novel From Here to Eternity, James Jones, actually appears in the scene where Sgt. Judson (Ernest Borgnine) is playing the piano at The New Congress Club. (Source)

The army agreed to collaborate with the making of the film in exchange for toning down the negative depiction of the army itself. For instance, in the book, Captain Holmes is promoted—but in the movie, he receives his comeuppance and is forced to resign. (Source)

Frank Sinatra didn't really get his role in From Here to Eternity by having his Mafia connections chop off a prize racehorse's head and put it in the bed of its owner, a stubborn Hollywood producer. The Godfather spread that legend. He (probably) got it through the movie connections of his then-wife, actress Ava Gardner. Bo-ring. (Source)

Parts of the Pearl Harbor attack scene in From Here to Eternity included actual war footage and filmed reconstructions, shot by John Ford for a 1943 wartime docudrama, December 7th. (Source)

In preparing for his role as Prewitt, Montgomery Clift took boxing lessons and learned to play the bugle—even though he knew a real bugle player would overdub the bugle parts. He just wanted to get that deep into the character. (Source)