The song "Some Sunday Morning", written for this movie, went on to be hit records for numerous singers of the 1940s, including Frank Sinatra, Helen Forrest and Dick Haymes.
Bozic in the film twice refers to riderless horses as "empty horses". This is likely to be a reference to director Michael Curtiz, with whom Errol Flynn had worked on The Charge of the Light Brigade (1936) (and whom Flynn detested). When wanting to see stray horses wandering through the battle, Curtiz directed the wranglers to "bring on the empty horses." After David Niven and Flynn cracked up laughing, Curtiz responded with, 'You people, you think I know fuck nothing; I tell you: I know fuck all". Niven later made this "Curtizism" immortal by titling his autobiography "Bring On the Empty Horses".
Errol Flynn's character, Clay Hardin, is a combination of the names of Clay Allison and John Wesley Hardin. Both were notorious Texas gunmen of the era.
Warning Spoilers: Tom Tyler (Leif) was also in "Stagecoach" with John Wayne. In both movies his character walks away from the gunfight after being shot, before falling dead.