Character voice is a subtle art. Certainly, it a bit of a chaotic one. You have to know enough about them to know what they might say in a particular situation, and if you think that’s not terribly hard, then bring to mind a person you know very well. A sister, a parent, a best friend. Has there ever been a situation where, when you were present with them, that you knew what they were going to say? Unlikely. The good news is, characters are, out of necessity, less complex than human beings. Understanding their essence takes a little less work. But that doesn’t make it easy.
How do you keep your characters’ voices consistent and distinguishable, especially with multiple point of view characters?
First, let me note that, for the sake of this discussion, I’m going to speak primarily about first person point of view narration. If you are or intend to write in third person narration, character voice will still be important, especially dialogue, but first person narration requires getting inside a character’s head so that you know what they think (or don’t think) and say.
That said, I should also note that, even in first person point of view, voice is not just important for the narrators of your story. Their voice will be the richest, although also the most difficult to set apart, but they’re not the only characters the reader will hear. To a certain extent, any character you have speak directly (or I suppose even indirectly) on the page will require a certain degree of attention to word choice and placement. However, these speaking characters — especially if they’re a very minor character and, by extension, talk less than the major characters — really only require keeping a head for the time and place. To rephrase: if there is a major difference between your narrator and the person/people they’re speaking to, make sure that it’s evident.
What do I mean by that? Well, let’s say your character is educated, and s/he is interacting with a character who is not. You don’t have to make the uneducated character seem dumb per se, but just take care with the vocabulary that you don’t accidentally slip some uncommon word into a character’s mouth who wouldn’t even know what that word means. Or, another example: let’s say you’ve got one character who wants to seem smart and one who is a self-assured intellect. The character who wants to seem smart will likely throw in a lot of big words when smaller words would have done the job just as well. The character who doesn’t care will likely use a handful of big words, but use them much more naturally, and of course will use smaller words if they do the job better than the bigger ones.
Likewise, do not forget location, culture, time, etc. There are plenty of common phrases that we might use in the modern age that a book character would have no reason to use. If your main character is very localized — say, a farmer in a more medieval setting — they’re not going say, “She’s happy as a clam” because even if they somehow know what a clam is, clams are not a big enough part of their lifestyle to come to mind so quickly. Of course, following the same logic, in that kind of setting, “You’re such a pig” could very easily be a common phrase for the character to throw at someone. So… essentially, mind the metaphors.
Now that we’ve got the dialogue down, we get a chance to discuss what I consider the fascinating process of internal thought. There’s a certain amount of writing first person PoV that’s really just instinctive. When I write, I don’t put myself in my characters’ shoes as some authors would suggest. But there’s a certain essence to any character that just sort of shines through. PoV chapters are an honor to bestow on a character, especially if they’re not a major character, so for the narration to flow well enough, you have to know where they come from, how smart they are, what’s most important to them, and so on. That’s primarily the “essence” of a character. That, and the requirements of the plot itself, drive your characters’ actions and often enough even their thoughts. It makes it easy to get your character — action and thought — through a scene if you know what that scene has to accomplish.
However, there will be more than a few times where you’ll reach a sentence and find that word choice does not come easy. You’ll find a moment of idleness, a time where the character gets a second to breathe and think, but what thought? And it’ll be then that you’ll want to refer again to the points I made in regards to dialogue. Who is this character? Is he educated, or uneducated? Or does he like to pretend he’s smarter than he is? What does he do for a living, and what did he grow up doing? What major experiences has he had in his past that have affected the person he is today? And, outside the character, what kind of technology is available at this time? What’s considered normal and what’s considered taboo? These questions you should already have answers to, or should at least have an inkling of what the answer might be. Then you just draw from those answers, depending on the relevance of them at that particular time in your narrative. Do that, and while the writing might not become magically easy, it’ll at least make it easier.