Yes, you read right. There's an art to creating those ubiquitous individuals known in every role-playing game as Non-Player Characters. It's not as if they just spring into being overnight (much as a Gamemaster might like them to...) This article is designed to help the GM with what often seems to be an insurmountable task -- creating and playing truly well-rounded NPCs. Take note that the information herein is concerned mainly with White Wolf Game Studio's Storyteller System, but much of this information can be adapted to most other game systems with little effort. That said, let's get down to business!
1. COME UP WITH A CHARACTER CONCEPT FOR YOUR NPC BEFORE YOU EVEN START PUTTING NUMBERS ON A SHEET.
Once you come up with the concept, the numbers will generally take care of themselves. For example, an NPC who's basically a fighter will obviously have high physical attributes, as well as a high Brawl score, and possibly a high Melee, Firearms and Dodge as well. On the other hand, if the NPC in question is a bookish, scholarly type, you'll want to concentrate most of her Attribute points in the Mental stats, and give her a high Research, Investigation and Expression score.
In creating my own NPCs for the Vampire game I GM, I've found that the best thing to do is come up with the character's concept (ie, what does s/he do for a living?), and then go over to the character sheet, marking down the Abilities that match the concept. My Gangrel private investigator has high Mental Attributes, with Physical coming in secondary, and Social tertiary. Most of his ability scores (Investigation, Security, Stealth, Firearms) reflect his occupation. Always keep in mind that the NPC's Attributes and Abilities should reflect his or her concept. If it doesn't fit, you'd better have a VERY good reason for it, or your PCs will start to feel cheated.
2. NPCs MUST HAVE BELIEVABLE MOTIVATIONS, JUST AS THE PCs DO.
I realize that I sound like a method actor, but it's true. One of the main things to consider when you're playing an NPC is "What is his Motivation? And do his actions fit with it?" If you've established that the NPC is a nice guy, don't go and have him pull sneaky, underhanded things (unless you can get away with saying that all that niceness was just an act!)
On the other hand, you might be able to have a truly nasty NPC suddenly do something nice (like let a fallen PC go when she has the upper hand), as long as it can be justified by her personality (perhaps she just likes to play with her prey....)
3. IN ORDER TO ROLE-PLAY NPCs WELL, YOU NEED TO UNDERSTAND THEM AND CARE ABOUT THEM AS PEOPLE IN THEIR OWN RIGHT (yes, even the bad guys.)
If you genuinely understand them, you'll be able to get into the role and be comfortable playing the character, thus affording yourself and the players a better, more entertaining time.
4. NON PLAYER CHARACTERS NEED TO HAVE THEIR OWN PERSONALITIES AND MANNERISMS; EACH OF THEM MUST BE A UNIQUE INDIVIDUAL.
Real people aren't made with cookie cutters; it stands to reason the people in a game world shouldn't be either. Besides, it's boring to both you and your players if all the NPCs look, act and sound alike.
Okay, so how do you make each of your NPCs memorable and unique without going crazy? I've found that borrowing a character concept from movies, books or TV can be very helpful in this regard. A Order of Hermes mage in my world bears a striking resemblance to Mercedes Lackey's Last Herald-Mage, Vanyel Ashkevron, one of my favorite heroes in all of fantasy fiction. (And for all of you readers who know of Vanyel's -- ahhh -- preferences, I can assure you that Andrew does not take after his inspiration in that regard.)
Also, the Storyteller needs to have at least some acting ability. If you can do accents, give it a whirl--don't worry that you sound silly; chances are your players won't notice (or if they do, they ought to have the decency not to make fun of the GM....). My Order of Hermes Adept, Dr. Julian Mordrid, has an accent based on Timothy Dalton's, while one of my Toreadors has a thick French Creole one. Let your imagination soar free!
While we're on the subject of acting, let me interject here that the right props can do wonders to help the GM get into character. They don't have to be elaborate; something as simple as a piece of jewelry can do it (I've found that wearing a pentacle necklace really helps me play Andrew Ryan, one of my mages.)
5. OLD PLAYER CHARACTERS MAKE WONDERFUL NON-PLAYER CHARACTERS.
If you've ever roleplayed on the other side of the table, take a look through your old character sheets. Chances are quite a few of your old PCs could make fine NPCs, whether "as-is" or with a little judicious "tweaking." One of my Toreador NPCs is a "revised" version of my very first Vampire: The Masquerade character (I added a few new things here and there, and married her to the aforementioned Gangrel P.I.) This also gives you a chance to resurrect your favorite characters (Don't you love being an all-powerful GM?)
Above all, relax, be comfortable, and HAVE FUN! Isn't that why you're doing it, after all?