At night, the actors would actually get together to play music and sing. When they were drunk, they'd make Lou Diamond Phillips sing "La Bamba", as he had played Ritchie Valens in La Bamba (1987).
When the men are going through the Indian village, "Doc" is in the front of the group, with a cover on his face. It's a stand-in for Kiefer Sutherland, who had left the set that morning due to the birth of his child.
In one scene, Billy reads a report that claims he is a lefty, and replies, "I ain't left-handed." This is a reference to films, books, and media wrongly claiming Billy the Kid was left-handed based on a tintype photograph of him. Tintypes produce a reversed image, making Billy look like he used his left hand to shoot.
None of the movie's fight scenes were choreographed. The actors just improvised.
Like virtually all movies about the events surrounding the Lincoln County War, John Tunstall is depicted as an older, sophisticated man. Tunstall was 24 when he was murdered, younger than most of the Regulators. Josiah "Doc" Scurlock was 31 at the time of Tunstall's murder, and Richard "Dick" Brewer was 27. Only 20-year-old Billy the Kid was younger.
Tom Cruise: the fourth person shot when Charlie (Casey Siemaszko) bursts out the house shooting, at around 1:39:18. Cruise was on set one day, and the director thought it would be great to get him on the movie. He dressed Cruise as a soldier, complete with false mustache and sideburns, and filmed the climactic battle.