What Happens Next - Or What Just Happened?
But first, let's define some concepts. The DMGII is quick to advise gamemasters to have a "story arc" but it never stops to define what a story arc is. We will: A story arc is a meta-story that links individual storylines together. Stories, of course, are as old as mankind, but not every story is part of a story arc. Consider the early Greek myths of Perseus slaying the Medusa, and of Bellerophon slaying the Chimera. Each has a story "line" but there is no link between the two. In contrast, consider the later myths of Paris wooing Helen of Troy, Achilles slaying Hector, and Odysseus encountering Circe - these are all linked by the "Trojan War" story arc. For a more recent example, contrast the adventures of Robert Howard's Conan (short stories without a story arc) with Tolkien's Bilbo and Frodo (adventures within a larger story arc).
Many people's earliest experiences of role-playing games are similar to those early Greek myths or Conan short stories. The party of adventurers goes to the dungeon, knocks down some doors, kills some orcs, and goes home. Next week's adventure, they knock down some thicker doors, kill some ogres, and so on. The first published RPG products that actually linked adventures across space and time was Gary Gygax's G-D-Q series of modules (Against the Giants leading to Vault of the Drow culminating in Queen of the Demonweb Pits. But the big breakthrough came when TSR gave us the Dragonlance series, which featured a story arc over a dozen modules long. James Maliszewski has described Dragonlance as gaming's Lord of the Rings, and with good reason.
To players in the mid 80s, story arcs must have seemed revolutionary, adding highly detailed backstory, depth, and a sense of purpose to what had previously been only loosely connected modules. And those are all wonderful things, things that any good gamemaster should strive to have in his games. But there was a hidden cost to the story arc: a cost in player agency. A story arc only works if the narrator can create a plot, that is, a sequence of events that effects change on the situation of his protagonists. If the narrator is a gamemaster, then the players are his protagonists, and he's made a commitment to effecting change on them. The players are now the objects, rather than subjects, of a story. A story arc transforms adventurers and agents into actors and audience.
For this reason, I call campaigns that use a story arc "directed stories." The gamemaster, like a stage or movie director, is directing the sequence of events that will occur with an eye towards achieving particular outcomes or expressing particular themes. It is story focused on what happens next. The opposite of directed story is emergent story, story focused on what just happened. Emergent story is the memoirs of your fictional characters, and the history of their fictional deeds.
A directed story GM is concerned with whether or not what the players are moving things in the direction the gamemaster desires. An emergent story GM is concerned with whether or not the players are succeeding in moving things in the direction they desire. A directed story GM spends time in between sessions working out what will happen next. An emergent story GM spends time in between sessions chronicling what just happened.
A directed story GM is a fortune teller who predicts that awesome things will happen to you in the future. An emergent story GM is a bard who weaves a story about the awesome things that you made happen.