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Tim Curry was originally cast as the Joker, before he was replaced by Mark Hamill. Both played villains on Criminal Minds (2005): Curry as Billy Flynn, a.k.a. The Prince of Darkness, and Hamill as John Curtis, a.k.a. the Replicator. Ironically, three of the main cast from Criminal Minds (2005) played iconic villain and hero characters, with Matthew Gray Gubler and Thomas Gibson playing villains The Riddler and Deathstroke in the animated movies Batman: Assault on Arkham (2014) and Son of Batman (2014), respectively, and Shemar Moore played the hero Cyborg in the animated movie Justice League: War (2014) (and various sequels).
Kevin Conroy is the first person in animation to use two distinct voices to portray Bruce Wayne and Batman, inspired by Michael Keaton's performance in Batman (1989), the first to do so.
Tim Curry was initially cast as the Joker, having also been considered for the role in Batman (1989), and it was long rumored that his portrayal was deemed "too scary." In reality, Curry had recorded four episodes, but dropped out after developing Bronchitis. The role went to Mark Hamill, who had already been cast as one-off character Ferris Boyle in Heart of Ice (1992). Curry ended up providing additional voices in a handful of episodes.
Dick Grayson, a.k.a. Robin, had to be older, because of censorship rules against showing the endangerment of children.
When establishing Two-Face, the writers took the time to first show his alter-ego, Harvey Dent, in several episodes, so the audience could get to know him better. That way, they were more shocked at his transformation from the upstanding Dent to the monstrous Two-Face. The writers believed this made him into one of the show's best characters, and his origin is considered one of the show's best episodes.
The animators auditioned over 500 actors for the voice of Batman before they settled on Kevin Conroy. Conroy has since become the signature voice of Batman in numerous media.