Voice is a huge part of character development. In "regular theatre" where an actor is only playing one role it's not so obvious, but in Fringe, where very often actors are playing multiple characters, it becomes increasingly important. I'm not talking about accents or funny voices, I'm talking about physically internalizing the character. It's essential. When actors use external indicators like costume to differentiate between characters it can be helpful to the audience, but if the actor doesn't believe the change, the characters all blend together despite the best intentioned visual cues.
The other thing is articulation. Young and inexperienced actors excited about singing or doing accents can forget about diction in their enthusiasm. All that energy is wasted if I can't work out what you're saying.
I have to mention Kitt and Jane by SNAFU Dance Theatre. Aside from the fact that I think this show was beyond awesome in a million other ways, it's pretty awesome vocally too. Ingrid Hansen is inspiring as the plucky Kitt and has a gorgeous(!) singing voice, but what really impressed me was Rod Peter Jr. as Jane. His thin, bright character voice was so solid that I was not expecting such a rich and resonant one when he opened his mouth after curtain call. Such an intelligent and healthy choice, directing his voice through his cheekbones and facial mask adding a thin, "weedy" quality without loosing any of his projective range. They could hear him in the back just fine even though it felt psychologically like his voice would disappear into himself at any second. Coupled with his introverted posture, the voice completely sealed the illusion. And by altering the direction of his voice instead of up-pitching, and keeping the breath deep and connected, there's no damage done so he can keep doing it night after night! I was very impressed. Might steal that trick myself some day. . .
Some great examples of multiple characters done really well were Paul Cosentino in Bad Connections? (also an AWESOME script by the way, written by Michael Levesque) and Andrew Bailey in The Adversary (which he also wrote, fit him like a glove). There were a LOT of shows in the Fringe and I didn't even see half of them, so this isn't a definitive list, just a sample of what impressed me on a purely vocal level.
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