The widow of credited Screenwriter Donald E. Stewart found the "Hostiles" script when moving houses. Stewart, who died in 1999, had not shipped the manuscript to any studios, and his wife was surprised to come across it. After seeing Crazy Heart (2009) and Out of the Furnace (2013), she felt that Scott Cooper was the right man to shepherd it towards the screen.
Rosamund Pike revealed in an interview that she and Christian Bale barely spoke during filming outside of shooting their scenes, saying that Bale "would keep a distance. Our characters in 'Hostiles' are quite dysfunctional, but have this profound connection that we let happen without ever really talking about it."
The film takes place mostly outdoors. As the movie was shot on-location in New Mexico and Arizona during monsoon season, the cast and crew were exposed to the elements. Indeed, filming had to be often shut down for a time because of lightning storms.
Christian Bale and Rosamund Pike were supposed to work together a year earlier, in a movie called The Deep Blue Good-By, which would have been directed by James Mangold, but the project fell through due to Bale's leg injuries. Bale recovered, but the project did not resurface and he teamed up with Pike for this film.
The book Captain Blocker (Christian Bale) is reading at the beginning of the movie is Julius Caesar's Commentarii De Bello Gallico (The Gallic War) in the original Latin. The page shown is from Book V, describing the social and economic structure of first century B.C. ancient Kent. In 55 B.C., Caesar invaded briefly the south of the England.
Wes Studi and Adam Beach have appeared together in other films, most notably as Detective Joe Leaphorn and Sargent Jim Chee in three movies based on books written by Tony Hillerman: A Thief of Time (2004), Coyote Waits (2003), and Skinwalkers (2006). They have also appeared in other films including Bury My Heart at Wounded Knee (2007).
This was the second time Rosamond Pike worked with an actor who played Batman. She previously worked with Ben Affleck in Gone Girl (2014). Christian Bale played Batman in Christopher Nolan's The Dark Knight trilogy and Affleck played the character in the DC Extended Universe films.
This film was filmed in chronological order, but there was one scene in the beginning of the film that was used toward the end of the film. At 29:09 after the Cheyenne men are chained they are ready for sleep, and it is this image of the family next to the campfire that was used again at 1hr 51 minutes. The group had just arrived at the Valley of the Bears and the family is resting in front of the campfire; only the lighting is a little dimmer on their faces.
The trivia items below may give away important plot points.
Christian Bale initially had reservations about the closing of the film, feeling it was "a movie ending", and would debate Director Scott Cooper on the matter. Bale felt that his character should just disappear into the crowd never to be seen again. Trusting Cooper's vision, he filmed the scene as written and came to appreciate the ending for its hopefulness, but also for the questions it raised about the future.