I'm really into mystery and suspense novels, so I see this happen all the time and I've just kind of accepted it as something that happens. But now that I'm writing my own stories, I've started using this convention with some characters... not all, but just some. I don't really have a rule for when I do it except for that usually if it's a person where the name is just a way of identifying the person (like if it's the boss, or the prime suspect in an investigation... I might use last names). Does anyone else have guidelines for when it's good to use last names and when it's good to use first names?