Character voice is a powerful tool for writers, and it is especially important when it comes to characterization. The voice, or tone, of a particular piece of writing gives the reader further insight into the characters the writer creates. You can't escape voice, even if you want to, because all stories are told by someone. There may not be a clear narrator (as is the case with many third person narratives), but there is always a voice, and that voice should be consistent throughout the piece.
Even though every story has a voice, it is easier to note when the story is told through the first person narrative. When a story is written from the first person point of view, you are reading the narrative of the character just as if you were listening to the character's thoughts. In first person stories, voice is usually easy to identify.
Third person narration, however, has far more variations, sometimes making it more difficult to notice the voice of the story. In some forms, third person narrative is just another form of first person, simply replacing I with he or she. Sometimes third person involves a neutral voice, one where the narrator is above the character, not inside the character's head. Third person can also mean dropping into the head of any character at any time. In many third person narratives, the voice of the novel is not consistent with any particular character, or may change to reflect essence of different characters throughout the piece, but that doesn't mean these narratives lack voice.
As a writer, you get to develop the overall voice of your novel. It is totally within your control. As such, it becomes a powerful tool in developing your characters, both major and minor. Too many authors overlook voice as it relates to character development, but it really is very simple. Start by checking yourself frequently. The story is not about you (unless it's an autobiography or a memoir). It's about the characters themselves, and the voice of your story should reflect that. Try to take yourself out of the writing and focus on the characters.
When you're writing, think about the characters who is closest to your point of view. If your point of view is first person, the character is obvious. Third person takes a little more thinking, but you are almost always closer to one character than any others. The focus character may shift if you're writing in third person omniscient, so pay attention to these shifts. A shift in focus character means a shift in voice.
Once you know who your focus character is, address the language you're using. Newer up in flowery language or lengthy words, but this isn't always appropriate for your focus character. The language you use should be consistent with the character in question. If your character doesn't even know what 'loquacious' means, he or she certainly wouldn't say or even think that particular word. Choose something more in keeping with the character you're developing.
This might seem like it applies only to dialogue, but that's not true at all. Your descriptions, your dialogue tags, and everything else should use the same grammar and vocabulary your focus character would use. To an extent, anyway. If your character speaks in gibberish, please clean it up. And don't make spelling errors even if your character would. The point is there shouldn't be a word your character doesn't know or couldn't pronounce, but your writing should still make sense. Let the tone of the story reflect your characters, but don't use that as an excuse for poor writing or horrible grammar.
As a writer, you best serve your story by taking yourself out of the equation. In most cases, the novel shouldn't sound like you're narrating it at all. Your voice, the author's voice, should disappear in favor of your characters. This is a simple concept, but it can be hard to accomplish. You'll have to review every sentence, look at them with unbiased eyes. I don't pretend this is easy, but your story will be better for it.
