Okay so you have a concept and a plan for an NPC; now we have to deal with the execution. What follows are a few tips and thoughts:

Relax

It's a little nerve-wracking to introduce an NPC who is important to your campaign and who you've given a fair degree of thought to creating. This is to be expected and generally there are two sources of fear in this situation. The first is that you aren't prepared. The second is that the player will react unexpectedly. I'll be honest with you, both of these are very reasonable fears and fairly realistic. However, once you start GMing it doesn't matter if you're prepared or not, because you've committed. Really no GM is ever truly prepared; it's a myth that it's even possible to be fully prepared. As for the player doing the unexpected, well, that's what you want to happen! The whole point of roleplaying is cooperative storytelling, which works best when everyone's contributing. If the player does exactly as you anticipated, then where's the fun in that? So in the end you're horrible unprepared and the player is going to totally demolish your plans, but that's how it's supposed to work.

Remember the Audience

The player is your audience. As you introduce and start running an NPC you can gauge your success by the player's reaction (or lack thereof). If things aren't working then shake things up. For example, you've created a villain for an adventure and wanted him to be truly menacing but instead he comes across as rather silly, well, then do something else with the character. Never stick to a failing approach unless you have a good reason. On the other hand, when you do something that works with the player then do more of it. If you're running an adventure with three secondary NPCs contributing to the story and NPC #2 really works with the player then maybe you drop NPC #3 and give his part to NPC #2. Adjust to your audience.

Words are Good but Actions are Better

Yes, you will deliver a NPC mostly by roleplaying his personality but actions are very potent. If you want someone to be a villain, then have him do something villainous, or perhaps have someone in his service do something villainous. Remember, PCs are unpredictable and when there is action they have a tendency to act. Having an NPC beat up an innocent will make the NPC look villainous but will also likely prompt the PC to trounce the NPC. The lesson here is that villainy has a wide range and you want to find an act that the PC will dislike but not so much that the PC will immediately act to right the wrong, thus undermining or even eliminating the NPC, who you may have intended to use over several adventures or even an entire campaign. The same is true of positive reactions; if a NPC does a good deed then this generates a positive reaction in the player, but once again if you set up a crisis for a NPC, then don't be surprised when the PC jumps in to save the day, depriving the NPC of the opportunity. This isn't all bad; honestly, bringing an NPC in with offers to help is even better than having the NPC doing something heroic alone.

Player Control

Depending upon the number of NPCs accompanying the PC on her adventures and the complexity of the system it might be a good idea for the player to run the mechanical aspects of the NPC. Thus in a battle the player will be running the PC and NPC companions. This has to be something the player would want to do and a lot of times players have more than enough on their plates with just their PC. However, there is a continuum to player control. On one extreme the player runs PC and select NPCs completely, including roleplaying them, but this is actually pretty rare. It's a lot to handle and more than a little awkward. On the other extreme the GM can have the player roll for the NPC occasionally at critical junctures. This might be a few attack rolls or a piloting check; something that is important dramatically and thus relies upon the NPC's skill and the player's luck.

The right continuum depends upon the nature of the campaign and the system. Thus it might be that the player controls the story/fate/hero points of an NPC companion or it might be the player controls an NPC's particular skill set like his psychic abilities, melee combat, or something definitive to the NPC. This can really help the player connect to a NPC and also take up some of the workload of the GM. Give some thought to what aspect of the NPC would be appropriate for the player to use and also if necessary some notes on the personality of the NPC in relation to that aspect. Really players are usually very good about running NPCs pretty true to their personalities even when that is a detriment to their PC.

As a complete aside, sometimes you might even let the player run an NPC during a side adventure. There will be times when an NPC really strikes a chord with a player and the NPC has a personality that's easy to roleplay, in these sorts of situations you can just run a separate little adventure. This can be a little awkward at the start but push through and it can be a really fun experience.

Keep Notes

It is important to keep notes and to record things that you might not remember down the road. These notes should only be as detailed as you need to jar your memory about how you roleplay an NPC. This won't seem that important when a campaign begins but when you take a few weeks off for the holidays or you table the campaign for a few months before returning to it then you'd wished you had kept notes. If you're using an accent or some other gimmick for the NPC definitely note this. Seriously, even if you are consistently running a NPC with no breaks, things can blur in complicated campaigns.

Reboot During the Adventure

Okay sometimes an NPC is a complete failure. The NPC priest you created for the PC as an adventuring companion comes across as a complete bore and the player doesn't want anything to do with the NPC. Now what? Introduce a new priest with a different personality, who is desperate for a chance at adventure. Amazingly this new NPC has all the same stats as the original NPC just a new and improved personality. This works with everything. The PC kills an important villain early in the adventure, well, his partner with similar stats steps in to fill the void and also be a little more cautious and cunning.

Okay that's all for this article. If you have any requests for future articles, just add them to the thread for this article.