If I run a GM PC, it will be in consultation with the players to fill a missing role in the party and it will be designed to not hog attention. It won't have "face skills" (Role playing skills). It will be created using the GM guide NPC rules so it will not be fully fleshed out like a player character in order to keep it simple to run and level up. Like a player NPC, in most cases, the players will determine what it does. So, bottom-line, it will only be if we need to round out the party and it will never be the center of attention.

Similar to a GM PC, there may be times when an NPC travels with the group or the players ask an NPC to travel with the group. I will make sure it doesn't become the center of attention, it will use the GM guide NPC rules, and players will determine what it does in combat. As a practical matter, the only real difference between this and a GM PC is that it might be added more for story reasons than filling a role reasons, and it probably will not journey with the party for very long, probably just 1-2 sessions.

If you can't make a session, we will run your character as an NPC if it is not convenient to leave it out of the session. Only I will see your character sheet, but the players will say what the character does, and I will click the appropriate buttons. Once we get to high level, if you have a very complicated character, I might make a version of your PC using the NPC rules to make it easier to run. You will be kept even on gear and XP.

To summarize, in all cases, the party will determine what the NPC does, unless I need to have the character do something in particular that reflects its backstory and personality. NPCs will not be the focus of attention.