I never really watched Batman: The Animated Series, but this oral history from Abraham Riesman is really great.

Arleen Sorkin (Harley Quinn): I could give you an impressive answer about how I did the voice, but I don’t have one. I read it and I just thought that that was the best voice for her in the moment. I don’t want to pretend that I am this woman with great range, so I picked the one that I could do easily, and it worked. Paul decided to make her Jewish, so I put a little Yiddish sound in there. At least we know the Joker isn’t an anti-Semite. It’s his only good quality.