Here’s a good post from Writers Helping Writers: 3 Ways to Infuse Character Voice
Vocabulary and the way a character speaks are the outer layer of character voice—the icing on the cake. Instead of trying to build character voice from the outside in, get under the character’s skin by revealing how they experience and interpret the story world from the inside out. …
The three ways, incidentally, are what the character notices, their opinions about what they notice, and what actually matters to them.
What you know is inside a room will almost certainly be different from what the viewpoint character notices. What gets noticed depends on who does the noticing. Everyone sees the world through the lens of their own mindset, a potent brew of knowledge, experience, motivations, goals, preferences, hopes, fears …
People are opinionated. They have beliefs, and hopes, and prejudices about virtually everything they encounter. …
I’m not sure it’s possible to have a truly reliable first-person protagonist. That specific person is going to have, by definition, their own individual point of view, which includes, yes, their beliefs and hopes and prejudices; their expectations about how the world works and what people are like; their own feelings about how people ought to behave and the standards against which they judge everyone … all of that is indeed beneath the vocabulary and syntax that the author gives that protagonist. Same with close third; everything in the story is seen through the perceptions and attitudes and beliefs and expectations of the protagonist.
Omniscient viewpoint allows the author to step back from the protagonist(s) and, by showing the reader everyone’s perceptions, attitudes, believes, and expectations, prevents the voice of any one character from pervading the narrative. Of course, if you take that far enough, the omniscient narrator then takes on life and acquires a distinctive voice too.
Syntax and vocabulary do matter, however. This is a somewhat different topic, or a different aspect of the same topic, but I was thinking about that recently, because I heard someone comment (as seems to happen constantly) that no one these days uses “whom.” Yes, we do, and you know who particularly uses “whom” in their casual, ordinary conversation? Your super-formal characters in your novels, that’s who. All mistakes and non-formal locutions become jarring if placed in the mouth of a character who ought to, and usually does, speak with a formal style. They are jarring because they do not match the established voice of the character or because they prevent that character from establishing a credible voice in the first place.
If the author can’t use formal syntax herself, she will never be able to write a character who is convincingly formal. That’s a painful limitation to impose on yourself, which is why authors ought to learn how to use “whom” correctly; and the subjunctive mood; and “less” versus “fewer,” and whatever other features we expect in formal English and don’t necessarily expect in less formal contexts or from informal speech.
… and while on this topic, I will just say that the worst offender is incorrect “him and I” constructions. I am thinking right now of multiple examples of important characters who should not make that kind of mistake, but do, and neither the author, the copy editors, the proofreaders, or the beta readers caught it. Or if someone caught it, they didn’t think it was important.
Wrong. It is important. The author should know how to shift into formal locutions no matter how casual speech is put together in everyday life and no matter how less formal characters might speak. Mistakes like this can do brutal damage to the voice of a character who ought not make this kind of error.
I therefore nominate this specific mistake as Top Mistake To Make Sure You Fix If You Are Writing A Novel. Here is a post about this error. Here’s another. And another. If you’re going to put this kind of error into the mouth of a character, fine, but you should do it on purpose because that’s the way that character ought to speak, not because you yourself don’t know the correct form.