Build your world

The World is everything that's true before the story starts: where the protagonist is, what they own, what they already know, and how the people around them feel about them. PARENK calls the starting version of this the Initial State - simply: what the protagonist knows, owns and believes when the Experience begins.

Some of what's true in your world should never change, no matter what the reader does or what the narrator improvises. PARENK calls these Protected Facts: what must always be true in your world. If your mystery only works because the killer's identity was decided by you and never by chance, that's a Protected Fact - it keeps the narrator honest even when it's being creative.

You don't need to describe every object in the room. Describe only what the story actually depends on.