Mark Hamill was not the first choice for the voice of the Joker in Batman: The Animated Series. As it turns out the original choice was Tim Curry. After recording a few lines the producers of the show felt that
Tim Curry's take on the character was a little too scary, so Mark Hamill was called in to take his place. It's funny because I just can't imagine Tim Curry portraying a clown that is too scary for children...
Gah! Okay, I must have blocked Pennywise from my mind.
Here's one other interesting tidbit from the show. Batman: The Animated Series was one of the only shows in which the voice acting was done ensemble. Most animated TV shows and movies have each actor record their dialog individually, combining the cast post-recording. During recording, Mark Hamill was the only actor allowed to deliver his lines while standing, to better enable him to infuse the Joker with the manic energy required for the role. Here is a clip of Hamill doing the voice of the Joker, albeit for the video game, Arkham City but it paints a picture of what it was like working with Hamill on the show: