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I know this answer is very late to the party, but I do not ever take a trait unique to a single friend. Ever! It seems like a shortcut to me, that would be harmful for the exact reason the OP put ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/32587 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/32587 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
I know this answer is very late to the party, but I do not ever take a trait unique to a single friend. Ever! It seems like a shortcut to me, that would be harmful for the exact reason the OP put forth: My friend would recognize it, his other friends would recognize it, and if I don't make that trait purely positive and that character a perfect hero, I risk resentment from my friend, and risk having him made the butt of jokes by our mutual friends. Isn't it a shortcut? Isn't it just laziness? Doesn't it prove a lack of imagination on my part, that I cannot for the life of me invent an entirely fictional unique characteristic for my character? Doesn't it show a severe deficit in my analytic ability and understanding of humanity if I cannot for the life of me come up with a "coherent" character unless I cast actual people I know in my fiction? With such a deficit, I have no business being a writer! My characters may have generalities in common with my friends, but so do thousands of other people. I am a heterosexual with homosexual friends, if I write a homosexual character I may draw on "realistic generalities" I have learned from my homosexual friends, about themselves or their own homosexual friends, but in a way I expect to be true of tens of thousands of homosexuals. I will make strenuous effort to not include any detail I think is specific to only one (or even a tiny minority) of the people I know. I want that character to ring true to any homosexuals that might read it, without insulting or invading the privacy or betraying the trust of anyone I know. As for one comment about "writing what you know," I disagree. I at times need to write about thieves, spies, ruthless killers, rapists, corrupt cops and politicians, world class athletes and geniuses and businessmen. I write about magic and medieval settings, space colonies and aliens. I don't know what any of those lives are really like, and never will. I can inform my writing with research, history, logic and inference, sometimes from people actually in those roles. So that my writing is plausible. That doesn't mean I know what it is like to be a prostitute: I do know what a few prostitutes have **_said_** it was like.