For me, voice is often subconscious. I just hear the character's voice in my head, and that's what I dictate. But occasionally, it's conscious, an effort to convey something about the character.
In my Bobo Buttons, Private Eye series, Bobo the clown detective and Ghoul the security chief don't precisely like each other, and Bobo's not sure why. They work together professionally. They're coming to trust each other more and more as the books progress. But there's a distance there. And Ghoul always refers to Bobo as Buttons. Never anything more comfortable than that.
The magician Mater Reese, on the other hand, sees Bobo almost as a son. Mater, who is childless, has a long history of "adopting" apprentices as they learn magic under him and then eventually leave for their own careers. And Mater always refers to Bobo (and many of his past assistants) as "youngster." For him, it's a term of endearment. (Or is his memory failing, and he forgets names? We may never know...)
At one point in this book, Bobo meets a teen who's working an information booth. He doesn't know the kid's name, so he calls him "youngster." That's not an accident.
Consistency in how characters relate to each other, in public and in private, is a useful tool for defining and reinforcing characters.