I realize that, in general, the idea is for player characters to move the action forward. However, some stories need NPC's with detailed histories and developed personalities, which a narrator doesn't necessarily want adopted out. In one of the games I created, the NPC's were characters I had developed and knew extensively. I needed them for the story, but I did not want other people playing them and changing them. But aside from their challenge cards, there was no real way to make them usable by anyone but me.