One of the easiest ways to nail down distinct voices when writing is to talk to yourself. Act like you're doing a reading. Feel their emotions. You may find yourself doing natural body gestures. Write those in.

Try to envision how they say it as much as what they say. Sometimes characters will surprise you and you'll end up saying different lines than you planned, ones that are better than the originals.

Yeah, hearing voices is usually not a grea thing. And writing what they tell you to . . . Well, I suppose think of it more as a writer being possessed by characters . . .