I lied. The next post in the series will be up tomorrow, but in the meantime, have a few thoughts that are near and dear to my gaming heart. By this I mean the oft-abused tool of the GMPC.
Raise your hand if this has happened to you: there you are, adventuring merrily in your setting of choice with your ragtag group of compatriots, when suddenly an NPC embroils him or herself into your ranks like some Clingy McGuffin with a character sheet. It is at this point that many gamers would probably chuck a Monster Manual at the GM's head, if it weren't for the fact that said GM provided this week's pizza.
This is probably the most common scenario when an NPC crosses that dreaded line to become a GMPC. Why do these characters cause such foaming at the mouth, anyway? Is there a use for them other than cannon fodder and pack mule? Should they just never happen, or can they be not only acceptable, but even liked?
The biggest danger of the GMPC is that by nature they are a waste of spotlight. The players want to be active, they want to be the protagonists, and they want to be the ones that accomplish the great feats that challenge them. If this weren't the case they would find some other, less-dramatic use of their time. Your players might upstage each other a bit and jockey for the spotlight, and that's alright. Will they appreciate you the GM, with your godlike control over the story, stealing the spotlight? Probably not! You're already the Game Master. You create the spotlight; you point it at will. If Will is your own character, this leaves the party watching you play a roleplaying game all by yourself and not sharing your toys.
Don't do this.
Even worse is the GM who decides to resurrect their Favorite Character Ever from another campaign as a GMPC. Giddy with power, they Mary Sue it up, making the campaign all about Their Super Awesome Guy and the other characters that just sort of orbit him helplessly.
Extra-special-recipe don't do this.
GMPCs should be considered tools. Not extensions of the GM's ego, not even Their Awesome Guy they used to play. Like any other part of the game, they should serve a clear purpose and get out of the way when they're not serving that purpose. If your players find their new partymate objectionable, write him or her out, and find some other way to fill the gap. Adding a few useful items to help smooth over the role that a GMPC would otherwise take is a viable method with the added benefit of letting the players find a few extra Shiny Goodies; just don't go overboard. Less directly, you might increase the party's access to services that a GMPC might provide; you could also tweak your plans such that those services aren't so vital to success.
But let's say, for conversation's sake, that you are hellbent on including a GMPC, and that your group has promised not to disown you if you don't muck it all up. To be credible the GMPC must be equally affected by misfortune, equally able to make mistakes, and equally able to look really, really stupid sometimes -- maybe even a little moreso than the party members themselves, though making them too far behind the curve will create a burden, and then you have a completely different problem. You will have a very fine line to walk between the two sides of the GM screen in order to play an appropriately-balanced character, and you should be extra responsive to player criticisms and feedback to keep that balance.
If your group is smaller than average, a GMPC can take care of unwanted party roles (besides that of Carrier Of Stuff, Emergency Food Source, or Extra Meatwall). As an example, if everyone wants a healer to come with them on a long-term basis but nobody wants to actually play said healer, you could add a GMPC with the wished-for skillset to the group. More fleshed-out than a simple generic statblock but not quite as powerful as a player character, such an addition might provide the occasional plot hook. This could work for any role in the party, as long as that role isn't the only role that can 'win' in a given situation. The GM's hacker needs the rest of the party to cover her while she tries to power down the electric fences. The GM's spellcaster's abilities can't affect the group of attacking monsters, but they can bolster their allies' strength. Give the party an equally vital and interesting conflict to resolve, even if the GMPC is 'on point' so to speak...or just avoid situations like this altogether.
There may be a time or two that the GMPC ends up with the spotlight completely legitimately, whether through the players handing it over (unlikely but remotely possible) or the dice decreeing it to be so. In general, use it, then pass it on. If it's something that the players really, really should be doing themselves (performing a killing blow against the BBEG, for example), you might consider fudging your roll. The biggest, badassest victories should in principle belong to the players.
In an interaction-heavy group, a GMPC might provide something social to the players: a friend, a family member, a lover, a rival. With a deeply plot-immersion-centric gaming group the GMPC, if played well, might be accepted as just another party member. Sure, it's not common, but it can happen. Some groups may have difficulty relating to a GMPC in this manner, however, for the simple fact that they share an actor with every other NPC and event in the world. If you happen to be playing online, you might wish to keep your GMPC separate from your GM self. Multiple chat client accounts or using different text formatting on a forum can give your players a visual way to tell you and your character apart.
I will be the first to say that I actually quite like GMPCs, despite having had some truly awful experiences with them. At one point in a campaign I was running, the party had as many as three GMPCs in residence in various capacities, four if you count that one of my players was transitioning into a sort of co-GM position. (One of these capacities was 'incapacitated,' but still.) It was a huge chore to balance and we did see fit to get rid of two of them for a long while, but by being very careful not to step on anyone's toes, we were able to essentially have our cake and eat it too. Granted, this is an extreme example and pretty out of hand! Our group at the time was very character-focused and willing to interact with them as though they were regular player characters -- most gaming groups would probably not put up with quite so many sockpuppets. That said, I feel that the GMPC can be a very powerful tool when used appropriately. I've thrown up a lot of general guidelines which I hope can help if you find yourself one way or another with a GMPC on your hands. Happy gaming!
I have had great success, by the way, in jumpstarting a dull plot moment by stabbing a well-liked GMPC just off-camera. I highly recommend this method if all else is failing.