When writers talk about finding a "voice," it's an odd conversation. Because "voice" is not something cut-and-dried like, say, dialogue or description or exposition. "Voice" is something much more elusive, hard to pin down, difficult to define.
And, for me, "voice" is two-fold. I think there's the "voice" of the author (which has more to do with individual style) and then there's the "voice" of individual characters (which can't be totally separated from the author's voice, but I do think it's a separate issue).
Today, I wanted to examine the latter - the "voice" of a character. For me, "voice" is a mixture of things - characters' personality "voiced" through their actions, their speech patterns, their background experiences, their belief system. "Voice" is sort of a package deal.
If, for instance, your character is a strong Southerner, then "voice" will not just entail a Southern twang. It will also probably entail Southern values, mannerisms, speech patterns, way of living. Not in a stereotypical way, of course, but inside that character would be ingrained the "essence" of the South. And that becomes part of the character's "voice."
Ultimately, characters' "voice" is just a bigger snapshot of WHO they are as people. If you can capture that in your writing (through dialogue, narrative, direct thoughts), then you've got ahold of their "voice." I think one way to test whether your characters have their own unique voice is to see whether all or most of them sound/act ALIKE. If they do - if in comparison to each other, the characters feel generic, or are difficult to distinguish from other characters (in dialogue, or in action) - then perhaps their unique voice isn't coming through strongly enough...
Here's a wonderful, creative way to create authentic voice for your characters: A Voice Journal. If you were to ask me how to have a strong "voice" for characters, my first piece of advice would be to know the characters WELL, from top to bottom: their dreams, their fears, their quirks, their education/family background. Everything. Then, the "voice" will come much more naturally. It won't be forced. I think that's what this "voice journal" does, essentially - it helps you to get to know your characters well, freeing up their natural voice.
So -- how do YOU find a character's voice? Any tricks of the trade to share?