Dane Dehaan was 31 years old during filming ( 32, upon release ) while the real Billy the Kid was shot and killed at 21 years of age.
Chris Pratt, Ethan Hawke, and Vincent D'Onofrio all previously starred together in another western, The Magnificent Seven. Pratt and D'Onofrio also starred in Jurassic World.
Vincent D'Onofrio and Adam Baldwin both starred in Full Metal Jacket, though they shared no scenes together in that film.
Dane DeHaan and Ethan Hawke both starred in the movie Valerian and the City of a Thousand Planets (2017)