John Ford kept Victor McLaglen continually off-balance (and thus in character) by getting him drunk, changing his schedules, verbally abusing him on and off the set and filming scenes when he'd told McLaglen that they were only rehearsing. For the crucial rebel court scene, the story goes that Ford reduced the actor to a trembling wreck by promising him the day off only to bring him into the studio early and extremely hung over, insisting that he spit out his lines. McLaglen was so furious with Ford over this that he threatened to quit acting and kill the director.
John Ford was concerned that the scene where drunken "King" Gypo goes into the brothel for Katie would not pass censors. The studio came up with the idea to "put the cats in hats"--that is, have all the prostitutes wear hats indoors, thus dissuading the censorship board from thinking they were prostitutes.
Dudley Nichols became the first person to decline an Oscar, turning it down because of union disagreements. Academy records indicate that Nichols had taken possession of his Oscar by 1949.
John Ford had been very impressed by F.W. Murnau's Sunrise (1927) and wanted to bring an element of German Expressionism to this film.
The day before shooting Gypo Nolan's trial scene, John Ford told Victor McLaglen that he wouldn't be needed the next day so he should take a break, enjoy himself and not worry about his lines. McLaglen proceeded to go out drinking--which Ford knew he would do--and the next day was forced to film the scene with a terrible hangover, which was just the effect Ford wanted.