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It is one of those things that are a matter of personal taste. Sometimes referring to a character by its surname implies things - eg that they are inferior or superior to their peers depending on c...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/4151 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
It is one of those things that are a matter of personal taste. Sometimes referring to a character by its surname implies things - eg that they are inferior or superior to their peers depending on context, that they are old schoolfriends from the type of school where people are known by their surnames, that they are mysterious and don't reveal their first name to anyone, and so on. While consistency is good, you could also have a character whose last name is generally used except by one other character, implying a higher degree of intimacy between those two. But yes, you can call them by first or last names, or a mixture, as you feel correct.