Article on Gnomestew by Robert A. Neri Jr. concerning the layering and creation of NPC’s.
The article touches on the five layers important for believable Non-Player Characters. These are: Archetype, Physical Features, Gear, Skillset & Personality. It also goes into how much detail is actually necessary based on how the Game-Master intends to use the character. It also goes into such areas as quirks, habits & vices, names, relationships, and motivation.