GM PRep

Ron Edwards provides a list of ways that GMs can prepare for a session, and it's a good breakdown of the common ways different RPGs demand to be played. Your game is probably making one of these demands (or an entirely different demand) even if it doesn't explicitly say so in the text:

"Linear adventures, in which the GM has provided a series of prepared, in-order encounters.

"Linear, branched adventures, in which the GM has done the same as above but provides for the players proceeding in more than one direction or sequence.

"Roads to Rome, in which the GM has prepared a climactic scene and maneuvers or otherwise determines that character activity leads to this scene. (In practice, 'winging it' usually becomes this method.)

"Bang-driven, in which the GM has prepared a series of instigating events but has not anticipated a specific outcome or confrontation. (This is precisely the opposite of Roads to Rome.)

"Relationship map, in which the GM has prepared a complex back-story whose members, when encountered by the characters, respond according to the characters' actions, but no sequence or outcomes of these encounters have been predetermined.

"Intuitive continuity, in which the GM uses the players' interests and actions during initial play to construct the crises and actual content of later play."

-from "GNS and Other Matters of Role-playing Theory," October 2001