Respect your prep

Between sessions, you'll make preparations (i.e. "prep"). You'll write up threats and NPCs. You'll create or choose dangers and discoveries. You'll think of encounters and design locations to explore.

Some prep involves possibilities: things that the PCs might discover, dangers that they could face, problems that might arise. This sort of prep lives in a quantum state; until you actually use these ideas and introduce them to the fiction, these ideas aren't true. They're just ideas—ideas that help you punctuate the characters' lives with adventure.

A lot of your prep, though, involves making decisions—real, binding decisions about how the world is. This includes stuff like a monster's instinct, moves, and HP, or how a location got to be the way that it is, or who killed the widow Maire. It might include decisions about what will happen if the PCs don't intervene, or how an NPC would react to X or Y. This type of prep is part of the fiction already, even if you haven't yet revealed it to the players.

Make these decisions carefully and thoughtfully. Take your time. Follow the guidelines in the upcoming chapters.

Then, during play, respect your prep. Treat it as fictional truth. Describe the world, portray your characters, and make GM moves with these decisions in mind. If you've done your prep well, you'll find that sticking to it not only helps you portray a rich and mysterious world, it also (paradoxically) helps you play to find out what happens.

See Prep for more on prep.