Major non-player characters (those that aren't being done up full-blown in the game system) can be built in two ways -- you don't need to use one method or the other exclusively:

1. The NPC fulfils a purpose for the story, either as a source of influence or income, as an aid to performing a specific sub-category of skills, or as an Agent (although, for Agents, see below).
2. Each NPC has a central trait (like Accountant, or Kung-fu cop) and two secondary traits. They also have a flaw. Basically, they are built like an Over the Edge character, except without the dice.