Jared Leto grew out his hair and beard for most of 2016 to prepare for the role of Nick Lowell and have the appearance of an American prisoner of war.
At one point, Japanese filmmaker Takashi Miike was slated to direct the film with Tom Hardy on the titular role. However, they both dropped out due to scheduling conflicts. Jeremy Renner was considered as a replacement before Jared Leto was cast.
The film's plot borrows heavily from Kinji Fukasaku 's "The Yakuza Papers".
The ritual that both Nick and Kyoshi performs in an apology to the leader of the rival Seizu clan is called yubitsume (alternatively called otoshimae). In the Japanese underworld, it is a way for a member to show contrition or make amends to a leader for an offense.The ritual is thought to have originated with the bakuto, itinerant gamblers who were predecessors of the modern Yakuza. If a person was unable to pay off a gambling debt, yubitsume was sometimes considered an alternative form of repayment. Yubitsume was a form of credit and reputation score.