For the duration of filming _The Purge_, Jason Blum had Ethan Hawke stay in his house in Los Angeles. While there, Blum and Hawke would watch western movies together and Hawke suggested that Blumhouse should produce a western.
K. Harrison Sweeney is no stranger to Westerns; a native of the Cowboy State, he also provided the voiceover and motion capture performance for Mr. Irish in RockStar San Diego's interactive Red Dead Redemption (2010) (2010 Game of the Year).
The film is actually Karen Gillan's 2nd western. In Doctor Who: A Town Called Mercy (2012), the Doctor (Matt Smith), Amy Pond (Karen Gillan) and Rory Williams (Arthur Darvill) arrive in the American Wild West of the 19th century.
The same year, Ethan Hawke starred in another western The Magnificent Seven (2016), which is a remake of the classic western opposite Chris Pratt. Chris Pratt had starred opposite Karen Gillan in Guardians of the Galaxy (2014) as Peter Quill.
Some of the cast have starred in comic book movies: Karen Gillan played Nebula in Guardians of the Galaxy (2014). John Travolta played Howard Saint in The Punisher (2004) and Burn Gorman played Stryver in The Dark Knight Rises (2012).
This film not only reunites actors Ethan Hawke and James Ransone after Cymbeline, but it also reunites the two actors with producer Jason Blum following the Sinister films (Hawke in the first, Ransone in both), as well as Hawke and Blum following Blum's executive producing job on Hamlet (2000).