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Michael Ansara, who voiced Mr. Freeze in the iconic, Emmy-winning episode Heart of Ice (1992), uttered the words, "Revenge is a dish best served cold." This line was called an "Old Klingon proverb" in Star Trek (1966), where Ansara once portrayed Klingon Commander Kang. He even threatened the Enterprise crew with the line, "You will die in the icy cold of space."
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.