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To answer your question, I have to talk about the difference between a plot and a story. A plot is a sequence of events that happen. A story is an arc of rising tension leading to a resolution. (Th...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/26311 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
To answer your question, I have to talk about the difference between a plot and a story. A plot is a sequence of events that happen. A story is an arc of rising tension leading to a resolution. (These terms are sometimes defined differently, and even exactly the opposite, but that is what I am using them to mean here.) You could have a plot about Joe going to work. He catches bus A, then transfers to bus B. At work he talks to Dave who suggests he take bus C and transfer to bus D. The next day Joe tries it and finds it gets him to work five minutes faster. This is a plot. It has events. It even has a problem and a solution. But it is not a story. It is not a story because we have no emotional investment in Joe getting to work five minutes early, and Joe had nothing at stake in his decision to follow Dave's advice. A story requires that Joe wants something, that it is important to him, and that it is difficult for him to get it. We have to sympathize with his desire (not approve, necessarily, but recognize it as a human desire, and Joe as a human person, and become engaged with his struggle). The question then becomes, how far will Joe go to get the thing he desires. When something frustrates his desire, will he give up or try again? Just how much is he willing to do, willing to sacrifice, to get what he wants? How much is he willing to bleed? And in the end, either he gets his desire, or he doesn't, or he learns the futility of his desire and achieve something more worth having. An arc of rising tension followed by release. Plot puts flesh on the bones of this story structure. But the story is the story of a person. The person whose story is the main arc of you novel is your protagonist, though there may be other characters with their own arcs. So if you have a plot but you don't know who your protagonist is, then you don't have a story yet. And since story is the master of plot, and plot must bend itself to the demands of story to create an arch of rising tension and a satisfying resolution around the desire of a character, you need to figure out your story and your protagonist before you get too far down the road of defining you plot. The heart of every story is X wants Y and can't have it because of Z. Find those elements and you have your story, and once you have your story, you know who your protagonist is.