A little bit to add to the content of The Care and Feeding of NPCs, Part I and Part II:
- If you can, watch Breaking Bad and look at the background characters whose names you never learn. They all seem to fit seamlessly into the setting. Breaking Bad is actually a masterpiece in terms of examining how secondary characters fill out a setting, and in terms of how they can interact with the primary characters (including shifts in function over time).
- Published adventures often include statistics for NPCs you would never need to stat out for home games. Unless you are publishing, you probably need to stat less than half of the NPCs in an average module. Fewer, if the module is The Village of Hommlet.
- That said, stat out anything and anyone you feel like statting out, or feel the need to have a statblock for.
- As with all prepwork, do what you need to do first, and then do whatever you feel like doing afterwards.
- You don't need to "create a mystery" for every NPC that you introduce, or even for every major NPC. What you may wish to do, though, is create a list of interesting things to learn about an NPC, and then link them to NPCs as the need arises in play.
- NPCs change over time. Let them get married, fall ill, have children, move, fall on hard times, gain windfalls, and die of sickness, old age, or misadventure. The goal is to have a dynamic setting, not a static one.