There were two different (usually uncredited) character actors named William O'Brien - William H. O'Brien and William J. O'Brien - and they both appear in this film (uncredited) as gamblers.
Writer Richard Brooks was originally penciled in to direct but was taken off the picture after Clark Gable was cast. He recalls he was told, "Well, now it's a Gable picture, and you can't expect to direct Gable.