There is a certain, somewhat predictable, dynamic to events, in stories and in life. Even if your story starts off in the middle of some action, that action is usually no more than a jumping off place for the story to start. Small or large, apparently unrelated events come together to create tension that builds as these elements draw inexorably closer to the climax and resolution.
History shows us the truth of this as a benign rule gradually (or not so gradually) shifts to an oppressive one. Small incursions on the rule of Law increase in scope and frequency until either the people rebel or tyranny reigns. It is as true in any other endeavor, if more subtle. Change at first is gradual, otherwise the people would never accept it. As it reaches a certain point, it cannot but continue on its course. On an individual level, we do not change overnight except in extreme conditions and we would not accept abrupt change in someone else, or in outside events without some questions – or a fight.
The old story of how to cook a frog comes to mind. If you plop a live frog into boiling water, it will (quite sensibly) jump out. If you put the frog into tepid water it will swim around and enjoy the pool. Then you gradually increase the heat. By the time the frog realizes he’s in trouble, the heat has him too befuddled to act. Frog is cooked.
In developing a story arc, this dynamic is present, by accident or design, in everything from romances to thrillers. How often are our characters caught up in a sequence of events that narrow the choices they can make and build to an outcome that those characters may never have chosen otherwise? The heroine may not have anything but contempt for your hero in the beginning, then an escalating series of events turn that completely around to where they, not only must rely on each other, but learn to trust and respect each other. Change happens in increments, usually small ones to start, like baby steps that allow you to run later.
That gradual change is necessary. A reader will not accept a sudden shift in attitude without good reason, anymore than a population will. As events draw the elements together the shift may increase in pace along with those events. The story heats up as the pace quickens. Twists and turns in plot may obscure the end for a time, while building tension. We are using the same dynamic that exists in real events to create that tension.
Each event in a story will escalate the tension slightly. Alone, they may even be insignificant, but together, in proper sequence they cause surges, like waves as the tide comes in. Each one coming a little farther up the beach and closer together as the story approaches its climax. Significant events in life are like that, both on a personal level and a global one. Something to remember is that each event will alter the perspective of the characters and the reader. This is what makes the change believable.
Real life is the same way. Each experience we have causes a change in perspective and perception that can alter the way we see both past and future events. That experience can be things that affect populations or individuals. The rise and fall of the Roman Empire comes to mind, as well as the series of events that led me to start writing in the first place.