Our DM has kind of an interesting problem, where he basically has no ability to determine how much prep-work needs to go into planning a session—and he has this problem in *both* directions.
Like, in one session, he went through the work of coming up with background info, personalities, names, relationships for all of like 13 different NPC Diplomats for a single scene, none of whom (or their countries of origin!) were mentioned again after that scene.
Then, in a different session, he apparently had to make up every single encounter on the fly because he apparently "couldn't predict the PC…