on Writers Helping Writers:
Almost every character should have their own voice—their distinctive way of communicating their worldview.
To illustrate, here are three lines from Harry Potter that reveal Hermione’s, Ron’s, and Harry’s individual voices, respectively.
“Don’t go picking a row with Malfoy, don’t forget, he’s a prefect now, he could make life difficult for you…”
“Can I have a look at Uranus too, Lavender?”
“I don’t go looking for trouble. Trouble usually finds me.”
Because Hermione believes in following rules, she regularly tells Ron and Harry to do likewise, and she’s often very logical about it. Ron, however, tends to be a little coarser than the other two and usually says comical one-liners. Finally, Harry, who is always associated with trouble, often has to defend and explain himself.