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.
This film was originally made to be released straight to video, but its popularity had grown so much after its announcement that Kevin Conroy and Mark Hamill would be reprising their roles as Batman and Joker, Fathom Events and Warner Bros. announced that it would be getting a special two-night theatrical release. This was the second DC Animation's film to ever hit theaters, with the other being Batman: Mask of the Phantasm (1993). It was also DC's third R rated film to be released to theaters, as the others were Watchmen (2009) and V for Vendetta (2006). It was also the first R rated Batman and/or WB Animation film released in theaters, and the second R rated Batman film released at all 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 that 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 Batman: The Killing Joke (2016) to capitalize upon Hamill's resurgence.
When Warner Bros. announced they would release this film with an R rating, making it both the first Warner Animation and first Batman-related film to not be rated PG-13 or lower, speculation mounted this was a response to the runaway success of the R-rated Deadpool (2016) film. However, the studio said that the reason for the R rating was that "Batman: The Killing Joke" was a violent, intense story that they did not want to water down, similarly to how they had been prepared to deal with an R rating for the earlier film Batman: The Dark Knight Returns, Part 1 (2012), before the MPAA gave it a PG-13 rating. Warner Bros. also felt it fit into the darker Batman universe after Batman v Superman: Dawn of Justice (2016), for which they released an R-rated Blu-ray and DVD version with thirty extra minutes of new, more violent material.
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 upcoming Cartoon Network show Justice League Action (2016).
When Batman brings up the profile of The Joker while in the Batcave, there are multiple references shown in the pictures. The fourth image on the top row is a reference to The Dark Knight (2008), where Heath Ledger's Joker is incarcerated by Gotham PD, seen wearing the Heath Ledger-style green waistcoat while hunched over. The fifth image on the top row shows a bleeding/dead Robin, a reference to the 1988 Batman story line "A Death in the Family," where The Joker kills the second Robin, Jason Todd. This story line is told in the DCAU film Batman: Under the Red Hood (2010). On the second row, third image, there is a picture of The Joker that looks like a revamp of the cover of 1940s "Batman #1," which was the first appearance of The Joker.
The filmmakers said that the only way to adapt the Batman: The Killing Joke (2016) graphic novel was to make it R-rated (rather than the typical PG-13 rating). This also marked the first DC animated film to be rated R.
Similar to The Dark Knight (2008), The Joker wants to prove to the people of Gotham how close they are to being like him. In this film, he states, "All it takes is one bad day." In The Dark Knight (2008), he says, "Madness is like gravity; all it takes is a little push."
The playing cards used in the game of Solitaire are a nod to Harley Quinn, with their backing decorated in her signature red and black diamond pattern, changed from the red grid pattern from the graphic novel.
When Barbra Gordon is using the new police software in the library, the words "Lex Corp" can be seen in the bottom right of the screen. This would suggest that it is Lex Luthor who has created and supplied this software to Gotham PD.
When Batman is looking at a computer screen with pictures of the Joker, there are references to other comics and movies with the Joker. Some being: the tropical setting in "Batman" (1989) in one of Jack Nicholson's Joker scenes, and in "The Dark Knight" when Health Ledger's Joker is sitting in a prison cell, a play on the Joker comic book cover "The Man Who Laughs", the laughing fish that appear in "Batman: The Animated Series", etc.
Mark Hamill admitted at a Q&A at star wars celebration in London 2016 that while he wasn't disappointed with the final outcome of the film, he wasn't as completely satisfied as he hoped to be. He also stated he had originally intended and would have preferred to have completed the story as an audio book, not a full animated feature.
Batman v Superman: Dawn of Justice - Ultimate Edition (2016) and Batman: The Killing Joke (2016) are two of the R-rated DC movies to not only feature Batman, but to also be released on home video within a week of one another.
Even with it's theatrical release lasting only 2 days, being July 25th & 26th 2016, this film still managed to have worldwide box-office earnings of $12.3 million. Which is quite a feat for a traditionally animated movie with a $3.5 million budget, which had prompted Warner Brothers to consider releasing future DC Animated films in theaters. One of these films being Batman: Return of the Caped Crusaders (2016).
There is a scene where batman is interrogating a criminal in an alley way right in front of a most wanted poster of the Joker . He asks the criminal if he's heard anything about the joker , and replies "I swear I haven't" Batman then replies " Swear to me!" This is a quote from Batman Begins where he says the same thing to Detective Flass in an alley way.
"The Killing Joke" was not the original influence behind the name of alternative punk/rock band Killing Joke. The British band Killing Joke was formed in 1978, while the comic was first released in 1988.
The trivia items below may give away important plot points.
The upside down room Joker fights Batman in near the end of the film is the same room the Comedian has his first conversation with his wife. This was made as a metaphor, meaning that the Joker's entire world had been turned upside down.
Many Batman fans were upset with the scene where he and Batgirl have sex; however, the notion of a sexual relationship between the two had been implied years earlier in the Batman Beyond (1999) animated series.
When Batman is in the Batcave looking at pictures of The Joker on his screen, there are scenes from famous Joker appearances in different media, including Cesar Romero's Joker's first appearance dressed as a clown in Batman (1966), Jack Nicholson's Joker from Batman (1989) Joker Products commercial, Heath Ledger's Joker sitting in a prison cell in The Dark Knight (2008), and the cover of the graphic novel "Batman: The Man Who Laughs" by Ed Brubaker and Doug Mahnke.
After The Joker tells Batman his joke at the end, both Batman and The Joker can be seen and heard laughing. The camera pans away just after Batman puts his hands on the Joker's shoulders, and then only Batman's laughter can be heard. This is left purposefully ambiguous, and it is not clear as to whether Batman kills The Joker or if he lets him live.
Some viewers interpret the ending as Batman killing The Joker by either strangling him or snapping his neck. Original writer Alan Moore had stated that Batman brings The Joker into custody "by the book" as Commissioner Gordon demands.
Harley Quinn, The Joker's sidekick, was rumored to be featured in this movie, but she does not appear. This would have been her second appearance in a DC Universe Animated Original Movie, after Batman: Assault on Arkham (2014).
On Batman's computer screen, Harley Quinn (new 52 version) is also featured on the bottom left side, while a horribly bloodied Robin (likely Jason Todd) is featured in the upper right, likely as a callback to the animated film Batman: Under the Red Hood (2010).
When Batman is questioning a thug about The Joker's whereabouts, the thug says, "I don't know, I swear to God," to which Batman replies, "Swear to me!" This is a nod to a similar exchange in Batman Begins (2005).
In the Batcave when Batman is looking at eight different monitors, mostly featuring The Joker, there is one screen featuring the second Robin laying dead after The Joker beat him to death with a crowbar. This image is mirrored to look just like it appeared in the storyline Batman: A Death in the Family (Batman #426-429).