Another example, the party had a water elemental stuck on them after they escaped from a car chase. The party decided to hole up in motel and one of the party announced they were taking a shower.
Given I'm actually trying to beat the players, which is something most GMs are told not to do, I have to tread /very/ carefully so as to not do it in an unfun way.
I think I heard the term a long time ago on a podcast. When, as a GM, you take anything the party speculates about the mystery and declare it to be true.
But getting back to the Mystery stuff... If I run a Sphagetti plot, it's impossible for me (through the villain as a tool) to fairly outmaneuver the players.
I have a very minimal, passing, awareness of Propp's theory, and precious little else and consequently have to get a /lot/ of milleage out of it when I try to talk about RPGs.
@Shalvenay I think, in narrative theory, you can talk about non-anthropomorphic elements as being characters if they serve the role a character would ordinarily play in a trope.