Which sci-fi superstar voiced Joker in "Batman: The Animated Series"?

Have you ever watched a really good voice actor, who you don't recognize as a cartoon character, as they speak in your favorite character's voice? It can actually be pretty unsettling.

And of course, it also seems like a pretty sweet gig. Apparently, it must be, because celebrities have been getting into the voice-acting game like wildfire lately.

But this person was lending their voice to animated projects before it was considered cool. Who was it?