He doesn't plot out his books so he doesn't have a roster of characters ahead of time. I've never asked him specifically about it but my guess is that he puts in a name on the fly so as not to interrupt the flow and if it doesn't work well later on, he changes it.
He may have mentioned that in On Writing but obviously I'm not remembering it if he did.

I know he mentioned that for books with a lot of characters, e.g. The Stand, he had to have a listing of the names to refer back to to make it easier to get them right as he continued with the book.
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