Help me get a handle on the NPC rules. The possible NPC classes seem very limited (Commoner, Adept, Expert etc.) I assume these are VERY generalized classes and any NPC can fit in there somewhere, and if he/she cant... the you must make them a full fledged character. Right?
The 'NPC' classes are just there to represent simple less potent characters. And NPC can be any class in the game if the DM chooses. They just often use those 'NPC Classes' for NPCs because they are much simpler and easier to handle, where as normal classes can be quite complex and difficult to manage if you have a dozen or so of them running around in a situation.
For instance, lets say I need a roughtly 5th level character to be a theif at only one place in the story. I could make him a rogue, but then I have 3-4 feats, and 2 talents to pick that interact with sneak attack, and possibly other abilities. If I make him an expert, I just have to pick the feats and some skills so he can do what I want him to in the story and move on. But the head of the thieves guild would probably be a recuring character of some importants, so he I would make a full rogue or maybe a ninja, and go from there, putting more time into the more important npc.
It's important to remember that NPC stands for Non-Player Character. Just because a character has a level or two in a core class does not make them a "full fledged character," if by that you mean player character. Just take a look at the NPC Gallery and you'll see that several of the listed NPCs have core class levels. Just taking a level or two in a core class does not revoke NPC status. Really the only difference between an NPC and a PC is who is running it. If the GM is running it, it's an NPC. If a player is running it, it's a PC. It's really that simple. The specific NPC classes are designed to do a lot of "normal" things, so a GM can flesh out a campaign world with barmaids and innkeepers, and such.
Edit: Ninja'd by Kolokotroni