Ok guys let’s put it this way.

A fictional character is not a real person. We allow the author to trick us into believing they are a real person for the purpose of entertaining us. 

A fictional character is a function of what they want. What they want becomes the plot of the story. 

For example, Sherlock Holmes is not a real person who wants to solve crimes because he likes mysteries. We are only pretending that’s what he is. The reason Sherlock Holmes the-fictional-character-construct exists and tells a story of solving crimes is because he is a function that wants to solve crimes, which creates the plot of the story in which a man solves crimes. Him getting frustrated in his desire to solve crimes creates tension, the speed at which he gets or is obstructed from what he wants, which is to solve the crime, creates pacing. The evolutions of his desires and how he changes as a person in order to achieve his desires, or change what his desires are, is what we call character development. 

A side character’s most basic job is to help develop the main character or the plot. A villain is there to provide obstacles which prevent the main character from getting what they want, which here is to solve crimes. In this case, the villain covers their tracks so Holmes needs to solve small mysteries to get to the big mystery of who did the crime, so he can solve it. 

A supporting character, like Watson, is there to reveal the character of Sherlock Holmes through dialogue with him and observation of him. He is there to reveal to the audience how Holmes is progressing in his narrative. Is he getting closer or is he frustrated? Is he changing as a person? Watson is also there to support Holmes in advancing the plot of solving the crime. As a less central character, Watson can have a subplot which is his own wants which create his own character arc, which is a less central and smaller version of a plot than the main plot. For example, maybe he feels inadequate beside Holmes’s genius, and over the course of the story Holmes relies on him at key moments, which bolster Watson’s belief in himself but overall it all contributes to the plot, which is solving the mystery that Holmes wants to solve.

Once the plot is solved and the character gets what he wants, which is to solve the mystery, the main story is over, which is what we would call the conclusion or denouement of the story. There may still be matters to resolve though, for example, like where the character development has left us. How have the characters changed? With the main plot resolved, we can resolve the subplots like how Sherlock Holmes has changed as a person, if Watson’s feelings of inadequacy have been answered, and if it’s a serial, what Sherlock plans to do next (in this case, likely going looking for another case to solve). 

A story where John Watson is the main character is very different. Then we’re talking about a man whose main desire is to observe another man solving crimes. He wants to follow Sherlock around. That is his function. But now it’s a different, less traditional story. 

Every time you change what a character wants or whenever you change who is the main character and therefore what is the central plot that needs to be resolved in order for the story to end, you have a different story with a different arc, different traditions, and different expectations and anticipated joys on the part of the reader. 

I read a Sherlock Holmes mystery for the joy of seeing a mystery solved, and if it’s not solved I will be annoyed. I don’t necessarily need to see the characters change by the end though. I read a story about Watson observing Sherlock Holmes to see how these characters change by the end, and anticipate the joy of seeing that change, and if they don’t change I’ll be annoyed. I don’t necessarily need to solve the mystery if Watson is the main character, I can leave it hanging because the story isn’t necessarily about the mystery being solved, in that case it’s just a subplot. That’s one value to making such a shift. These stories can serve side by side, and even nest in one another, but they are different depending on who is the focus.

Good stories do a very good job of convincing us that we are seeing real life. We are all instinctive storytellers because we know how to take the chaos of real life and set it to a pattern that makes it all somehow make sense. It’s how we all don’t go crazy because of the entropy of the universe. But that story pattern itself has its own logic and language. In understanding the very basics of those, we can make and break those expectations to tell more interesting stories. Most storytellers are likely doing that right now, without even realizing it.