Zappo
Explorer
Sometimes. I feel that the best way is not going overboard with it; people can and do perceive surprisingly small differences in intonation, and big changes tend to become silly rather quickly because I'm not good enough to do a radically different voice. Also, you have many more features to work with than just pitch and accent; when I want to give an NPC a unique voice, I also slightly change my posture and body language. The longer the speech, the smaller you can make your changes, but most people can sense rather fine details in just a few minutes.