Our story takes place in a world. That world is either changing, or it isn't changing. This sounds simple. Don't worry: it gets more interesting! Also, understanding the changing or static nature of our world is important to developing a plot with significance and focus. Let's take a look at some examples:
A Changing World
A Static World
As is clear from these examples, an unchanging world can exist within a changing one, and vice versa. Many, many stories have been written about two or more characters who live in interlapping worlds, one changing and one stable. Perhaps one generation lives in a feudal village, while the younger generation has traveled with an army and seen crossbows slay knights. Alternatively, perhaps society at large is rigidly homophobic, punishing discovered GLBTers with public hangings, while within one home, parents learn to accept a gay child.
When we consider whether our story's world is changing or static, we are dealing with setting. Setting should not be confused with plot, which is the events that happen in our story. Events can intersect with our world in one of the following basic ways:
A Changing World
A Static World
So now we have ourselves a fine mess. The different characters in our story may live in somewhat different worlds, of varying size and stability, and then stuff happens to them that may interact more or less with their worlds . . . How do we keep our focus on telling a good story? I suspect that many writers who have trouble bringing their stories to an end, because they keep branching out in ten directions, have trouble getting a grip on this interaction of their plots with changing or static worlds.
A historiographical model may help us out. Historiography is the way historians decide why things happen. Different historians have adopted different models for why things happen, and they argue the pros and cons of their favored models at length.
One such model is that used by Fernand Braudel in his seminal work, The Mediterranean. Of the Annales school of historiography, Braudel believed that time is not one stream but three: geologic time, social time, and individual time. It's at convergences of geologic, social, and individual cycles that interesting stuff happens.
So how does this help us? Most characters, except for the most long-lived entities, will perceive geologic time as static. Stories set in geologic time are, from the point of view of most characters, set in a static world. The geology and climate of a world set up its possibilities, but they don't generally change within one's lifetime. Note that science fiction plays with this: terraforming, global warming, and an asteroid's impact are all ways in which geology/climate move from geologic time into our more immediate timestreams -- social or individual time.
The passage of social time can be perceived over a generation. To the extent that a story deals with a "generation gap," the characters are living in social time. Events such as the deterioration of feudal authority in favor of the crown, the tendency for grown but unmarried children to leave for the cities instead of staying at home, and a growing acceptance of Martian currency on Earth would all happen in social time. In the 20th-century developed world, social time cycles sped up dramatically, so that individuals could place themselves as leading or trailing broader socioeconomic movements.
Which brings us to individual time. Our characters, if they have human lifespans, readily perceive individual time. One nation declares war on another. A talented writer decides to take a year off from school and hitchhike. A shaman chooses a boy child born on a blue moon for his successor. Events -- our plot -- typically happen in individual time.
Most story worlds are created in geologic or social time. Most plot takes place in individual time. When these cycles intersect, we have a significant story on our hands. So, from each character's point of view:
Think about a significant book or short story and answer these three questions. Is the setting in geologic, social, or individual time? Does the protagonist perceive this setting as changing or as static? And does the plot happen in geologic, social, or individual time?
For example, let's consider Maureen F. McHugh's Mission Child (first chapter), about a girl in a kind of Laplanderish culture to which modern technology is being introduced. I see the setting as being in geologic time: the character roams the planet, which has its own rules for what humans can eat and how they can make a living. I see the plot as occurring in social time: the story develops over a decade or so, and thanks to the introduction of new technologies, social change is happening at a rapid clip. It is these social-economic-cultural changes that form the movement of the novel, rather than actions of the protagonist.
Now think about one or more of your own stories, and ask the same questions.
Okay, now that we've identified the time streams of our setting and our plot, let's go back to the interaction of events with our world. If our world is changing, are the events part of that change, or do they run counter to it? If our world is static, do the events disrupt the status quo, or do they reinforce it?
If the answers to any of these questions are muddy, then it's possible that we need to do more work with our worldbuilding. Without a clear understanding of our setting, our plots can lack significance or focus.
Recommended Reading on Historiography: