The 1988 graphic novel, written by acclaimed comic writer Alan Moore, is often considered to be one of the best Batman stories ever written, but had not yet been turned into an animated feature unlike Frank Miller's Batman: Year One or The Dark Knight Returns.
Speculation was running high that Kevin Conroy and Mark Hamill would both be brought back in to reprise their performances as Batman/Bruce Wayne and The Joker respectively, as they had done throughout the aforementioned Batman: The Animated Series, Justice League, as well as the critically lauded series of Batman: Arkham games.
Half of these hopes were dashed though when on January 10th, 2016, Kevin Conroy confirmed via his official twitter that he was not involved in the plans for The Killing Joke.
Timm recently stated in an interview with ConTV that the film was set to premier in July at the San Diego Comic Con event; a report that was reprinted on comicbook.com.
It's doubtful this far into production that the voice acting hasn't already been recorded, suggesting that if Conroy hasn't recorded lines yet, someone else was hired for the part.
The tweet was sent to Hamill as well who has yet to respond but had stated in several interviews that he would be happy to voice The Joker in an animated version of the story.
At least the two can still be heard exchanging barbs and taunts in 2015's Batman: Arkham Knight video game.