Characters are the most important plot element because they move the plot along. A plot that doesn’t move is a bad plot. A good dynamic plot is crucial to good story telling and characters, with their foibles, beliefs and attitudes can make the plot hum along to the end.
The plot depends on the characters. It is what the characters do, that determines the plot direction, and how it behaves. If you make your main character too much of a wimp, it won’t climb the mountain that is crucial to the ending. If you make your characters too stereotypical, the ending won’t be much of a surprise. Interplay between characters is also important. Even when you think there is only one character in the story, say the person climbing that mountain, you actually have two characters, the person, and the mountain. The person climbs a few feet; the mountain throws a rock or two at him/her. That’s how drama works. Will the person make it to the top of the mountain? Who knows? That’s pacing. The characters must be real enough to make the outcome doubtful. Maybe the character makes it to the top of the mountain. Maybe it plunges to its death. Either way, you have a story.
Characters can make or break a plot. If they are too smart, the outcome comes too soon. If they are too dumb, the plot stumbles along until it ends, whimpering. As for Goldilocks, every character has to be just right for the part it plays in the plot.
If the character doesn’t logically do what the plot demands it to do, the plot can’t move. The character has gummed up the works and there’s nothing else to do, but scrap the plot, or the character and start over. Like a performer on a trapeze, a character has to have its timing just right. If it makes its move too soon, the plot is not where it needs to be and the character falls to its doom, or the net, either way, the story ends too soon.
This is an issue when authors don’t get to know their characters before dropping them into the plot. The characters don’t get a chance to learn what they need to do in the story. The plot starts whipping at them, they don’t behave the way that they should because they have been miscast. Maybe the sidekick should be the hero/heroine or even the villain. I have seen heroes that I didn’t like and even sidekicks that made me wonder why the hero kept them around. Those stories suffered as a result.
Good character casting makes for a good plot. Authors need to consider their characters carefully when casting them into a plot. The characters and the plot need to match perfectly or the entire structure of the story fails. Characters need to make good decisions at the right time to move the plot forward, or bad decisions at the right time to hinder themselves or others to keep the plot moving at the right speeds. Characters can accelerate the plot or put the brakes on to keep the plot on the right road and keep it from careening off into the forest or over a cliff. That’s why characters are the most important element of Plot.