How do writers handle the player character(s) entering the game world in their stories? Do they consider the character to suddenly just exist in the world or are they established beings with lives prior to the PC taking over? If the latter I think it would be interesting to see stories where the PC has to deal with people who knew them prior to taking over. Actually, I am sure someone may have already done this, I have not read as much as I would have liked. I have seen where the character comes into the world and has to fight the impulses of the character to see who would be the dominant as far as the story goes, but it is usually resolved way too quickly with the PC winning out without much difficulty.
For me personally, it would depend on the story itself. In some cases, they might "take over" or "be put" in the body of a character/NPC in the story (like I could see a Dungeon series being like that? Fighting between the person and the desires of the dungeon?). In other cases, they might be teleported and thus the body comes with them.
Depends on the story, most of the ones I've read establishes the connects between the PCs and the MC in both the game and the real world. Some of them even have arcs for both however it's rare that anyone other than the MC would have any effect on the game. I mean... players can range from noobs to triple A pros with the majority of the population somewhere in the middle. makes sense for each to act accordingly.
Continue Online had a section of the first book where the MC was put into the body of an NPC and had to act according to what the NPC would’ve done. It was an interesting bit and one of my favorite parts of that particular book.