Sit still in a swing, and it is a bit boring. Start moving, pumping with your arms and legs to move in an ever higher arc, and it becomes a thrill ride – at least until you become tired. Then let the swing glide under its own momentum for a while until you feel like kicking it into high gear again.
The plot of a good story works in much the same way. Sometimes we show students the simplified version of a story arc. The exposition at the beginning is a flat line until the rising action ascends to the point of the climax and then the denouement is downhill from there.
Actually, I have read excellent stories that started with a steep incline in action (like receiving a strong push from behind the moment you sit on the swing), with the exposition filled in later as the energy of that initial push leveled out. Then, there were several ups and downs, before one last high reaching peak for the climax.
So in reality the action in a story should rise and fall like a swing. Although, it is not that simple either. For example, how many times do you go up and down? Dozens of times on a real swing, but in a story you don’t want to make your readers dizzy. I recommend to students writing short stories (2500 to 3500 words) that they chose one or two events to create rising action in the story before hitting the high point. But I would not dizzy the reader with 25 to 30 events that rise and fall in a 50,000 word novel. That is where the writer’s craft in creating a slowly rising tension comes into play.
Often we connect action with pace, or how much excitement/tension is occurring. However, in a plot it serves another purpose. The closer the character seems to be in resolving the main conflict, the more the action rises. Main character tries to climb mountain, makes it half way up – action is rising. He falls down, breaks a leg and is sent back to base camp – action is falling. The pace can quicken during falling action as well as rising action, but a continual habit of increasing pace without taking plot anywhere can make a story seem forced – exciting but empty.
Finally, there is the consideration of whether the problem will be resolved happily with success or tragically with failure at the climax. The swings in action will still occur, but the change in pace is usually not as great in a tragedy. The failure is often a slow decline, rather than a swift collapse. The writer must foreshadow to let the reader know the tragic end is coming.
Trying to create a story without considering the events that cause the rise and fall is like trying to swing without using your arms and legs to pull and push. The breeze will blow you a little bit, maybe.