On the Extra Criticum blog, Gary Garrison (dramatic writer and Executive Director for Creative Affairs at Dramatists Guild of America) has an interesting exercise for playwrights and screenwriters:
1. Take a page of dialogue from one of your plays that has three characters or more speaking to one another.
2. Using liquid white-out, blank out all the names.
3. Make a Xerox copy of that page.
4. Hand it to a fellow playwright, director or actor someone who reads a lot of plays.
5. Ask: how many people are speaking on this page? If they can’t tell you the number of people speaking, there’s a problem, no?
I like the simplicity of this, as a way to ensure distinct voices. Just don’t forget to re-add the names before first full cast read-through.