Back in the day, there was a commercial bulletin board software called TBBS ("The Bread Board System"). It differed from most BBSs in that it could handle multiple lines on a single computer, through a built-in multitasking implementation. Most TBBSs allowed new users to dial in and create new accounts by entering their name and choosing a password on the first login. TBBS had a companion product called TDBS ("The DataBase System") that allowed writing interactive programs for use by users. One of these programs that became popular was a light adventure MUD called Illusions. I had a two line demo of TBBS/TDBS I got from somewhere (I think legitimately, actually), and I downloaded a demo copy of Illusions to play with on my own machine. Looking at its databases, I found something curious - the players' characters and the non-playing characters were stored in the same database, with the user name for all of the NPCs being "aa non-playing character". It turns out the game did not check for a user using that name. And of course TBBS knew nothing of Illusions's design choices. And so by creating a new login account so-named, I was able to play as the NPC characters, moving them around town and chatting with other players to much amusement. At least until the sysop realized what was happening. That is all to say, I too have had the experience of choosing to play as a non-playing character.

(another fun fact: the companion product for connecting TBBS to the nascent Internet was called an IPAD - Internet Protocol ADapter)