From NYT bestselling author Dave Farland:
Many authors write tales where every character sounds the same. Everyone in their stories seems to be college educated, just like the author, and everyone seems to come from the same area as the author. That can be a real weakness in your story.
If your characters don’t have different voices, your story will never come to life. It’s so important, that recently while looking at the websites of several agents, I noticed that about half of them mentioned “voice” as the first thing that they look for when judging a tale, and I have to admit that when I pick up a story with a good strong voice, I instantly breathe a sigh of relief, feeling that I’m in the hands of a real pro.
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So give each character in your story a distinctive voice. You don’t have to be in-your-face about it. The distinctions might be slight. For example, two neighbor boys, even though they are the best of friends, might use slightly different slang. One might say “whoa” when an alien ship crashes into the woods, while the other says “awesome.”
One technique that I find helpful is at the end of each story, I go through in editing and make a “dialog pass.” If I have a character, Bron, I will search for instances of “Bron said” so that I can pinpoint his scenes and make sure that his dialog sounds like it is coming from him. This needs to extend beyond the level of just dialog. It might force me to ask questions like, “Is this what Bron would really say in this situation? Is that how he would say it? Would he really think that? Is this how he would react? Is this what he would notice?” In other words, part of editing his voice is also to reexamine every aspect of him as a character.
Link to the rest at David Farland