After the release of Batman: Arkham Knight (2015), Mark Hamill stated that due to the strain it put on his vocal cords, he would never voice the Joker again unless "Batman: The Killing Joke" was adapted into a film.
Mark Hamill had originally stated that this would be his last time voicing the Joker. Kevin Conroy personally contemplated on retiring his performance as Batman as well. However, they both changed their minds after they agreed to reprise their roles in the Cartoon Network show Justice League Action (2016). Hamill has since said that he'll continue to voice the Joker for as long as Conroy continues to voice Batman. After Conroy's untimely passing in 2022, Hamill announced that he would never voice the Joker ever again.
This film was originally to be released straight-to-video, but as its popularity had grown so much after the announcement that Kevin Conroy and Mark Hamill would be reprising their roles as Batman and the Joker, Fathom Events and Warner Bros. announced it would be getting a special two-night theatrical release. In doing so, it became the second DC Animation film to ever hit theaters (the other being Batman: Mask of the Phantasm (1993)), DC's third R-rated film to be released to theaters (the others being Watchmen (2009) and V for Vendetta (2005)), the first R-rated Batman and/or WB Animation film released in theaters and the second R-rated Batman film ever released since the Ultimate Edition of Batman v Superman: Dawn of Justice (2016) on Blu-ray.
Mark Hamill had previously announced his retirement from portraying the Joker after completing work on the "Arkham" video game series. He had stated the only way he would come out of retirement was if "The Killing Joke" was adapted to film. After the revival of the Star Wars franchise, starting with Star Wars: Episode VII - The Force Awakens (2015), thus reviving Hamill's popularity, Warner Bros. Animation immediately green-lit a "Killing Joke" adaptation to capitalize upon Hamill's resurgence.
The Joker's cell number, "0801," is a code in numerology that spells out "HA."