I’ve been spending some time thinking about the impetus for change which often kicks a story off the ground and gets it rolling.  Some change is essential to adventure or character driven stories in which action will play a key role, but a balance must be reached between catching the reader’s attention with a wild new thing, and forming believable change within the context of the world created.
The introduction of a stranger is frequently used. The death or departure of a loved one is also common. Some change within the character–a coming of age, or discovery of new abilities–can also work well.
What I’ve been playing with lately is the idea of change as an outgrowth of cross pollination between cultures. Situations such as: a previously nomadic tribe must settle near a watering hole due to drought conditions, and are affected by the people already in residence there. Or men in a hunting party see boats out on the ocean. The boats sail close enough to land that the men see a few details before they are gone.
The second scenario, especially, represents only a brush between cultures, yet it is easy to see the potential change which the introduction of a new concept can have on the individual lives of the story’s characters.
My thought is that the more natural the prompt for change the more deep-rooted the momentum, and therefore the more compelling the tidal wave which can grow from it. Giving change a nudge which is true to the world in which it takes place will also keep intact the internal consistency so essential to believable world building.