Great post ! I've got a few NPC's that have been kicking around and developing for a very long time now.

One in particular came to the fore a couple of years ago, she was very much the fixer type in the town she is based in. The party passed through, spent a lot more time there chasing red herrings than expected. I thought it would be a shopping and re-supply stop that would take about 15 minutes of play; they ended up being there for nearly three months.

Now, one of the characters gets a tick every time the name "Merielle" comes up, growls and says she'll go back and settle scores one day, and the player isn't much better. I'd say that's job done.....