"Voice" is difficult for me, but I've found that the best way for me to prevent my characters from sounding like the same person wearing different hats is to start with a real conflict between them. What people say tells you how they think - so in order to know what someone would say, you have to have a good handle on how they think first.
Make your characters opinionated, even if they only have bit parts! There's no such thing as NPCs in real life, so don't take the shortcut of pretending they exist in your fiction.
A word of caution, though: don't let your side characters upstage the main plot. This is what happened to Pirates of the Carribean: Jack Sparrow was so perfect in his supporting role in the first film that he got shoved into the role of the protagonist in the later ones, to the series's overall detriment. Some characters really do work better when they're not the protagonist!
