We all do it a bit to each other. This is true. We fill in with what we think we know and what we think we can reasonably infer.
But the construct moment that was most painful to me was when I realized that I had become this type of auctorial construct to some of my oldest friends. It was several steps out from "I fill in with what I think I know about you" to "you exist for my entertainment." And it was a lot more painful than, "X thinks I think this? But I don't, and he's known me long enough not to think so." Auctorial constructs aren't always treated like dancing bears, but there's a large amount of that involved in it. It's the line between, "Oh, pnkrokhockeymom is so funny!" and, "Be funny for me. Now. Hey! I thought you were funny! What are you doing being all serious like that? I wanted someone funny."
This doesn't just happen to fiction writers, obviously. But I think seriously asymmetric communication enables it--whether that's with music or fiction or a particular type of blogging or newscasters or God knows what.