According to director Antoine Fuqua, Martin Sensmeier was cast as Red Harvest because he auditioned with luxuriant, almost knee-length hair. Sensmeier wasn't told his hair was a selling point, and he cut his hair soon after. Fuqua was upset, then got the idea for Sensmeier to have his hair cut into a Mohawk.
James Horner worked on this film after he and Antoine Fuqua became close friends while making Southpaw (2015). According to Fuqua, Horner's team visited him on the film's set in Baton Rouge, Louisiana, one month after Horner's accidental death, to deliver the completed score. Horner had been so inspired after reading the script that he composed the entire score during pre-production.
Denzel Washington and Chris Pratt were the first two actors cast. Antoine Fuqua knew that both men had expressed interest in appearing in a western. Getting Washington was easy, but Fuqua initially was unsure in which role Pratt would fit. On the second phone call between Fuqua and Pratt, the latter started to sing "Oh, Shenandoah", which Fuqua immediately declared that "Pratt is Steve McQueen".
The characters use explosives produced by the Giant Powder Company of San Francisco. The company began operations in 1868, as America's first manufacturer of dynamite, under exclusive license granted personally by Alfred Nobel, the inventor of dynamite.