Protagonists are the reader’s conduit into the story — without them, the story pretty much wouldn’t exist. Often I think there are two ways writers come up with characters: either the characters come first to mind and you then figure out what story to build around them, or you come up with a story and you then figure out what character to put into it.*
Either way, at the end of the day what you want to end up with is the right character for the right story (or the right story for the right character). There should be a reason *this* character has to tell (or experience) *this* story. There’s an interlocking relationship there. If we can pull out the character from a story and replace them with someone else without affecting the story, then the relationship between plot and character isn’t strong enough. The same is true if we place you character in an entirely different story and it doesn’t affect the character’s arc.
Replacing the character should fundamentally change the story; replacing the story should fundamentally change the character.
For example, take two characters and two stories: Bella Swan from Twilight and Buffy Summers from Buffy the Vampire Slayer (two female teens in a world with vampires).** If we put Bella in Buffy the Vampire Slayer, it fundamentally changes everything — not just Bella as a character, but also the story of what happens at Sunnydale High. Suddenly, Bella has to learn pretty quickly how to hunt and kill vampires if she wants to stay alive long enough to fall in love with one. And I’ll just leave it to your imagination what happens if we send Buffy to Forks (here’s a hint: https://www.youtube.com/watch?v=RZwM3GvaTRM).
But even when both of those characters are in almost the exact same plot — mortal human falling in love with a vampire — they do so in very different ways. The challenges are different, how they respond is different, how it forces them to change and grow differs.
So if you’re someone who comes up with character first and then search for the right plot, some things to think about: what’s unique about this character — skills, feelings, quirks, desires — and what kind of plot can uniquely take advantage of those? What does this character want more than anything else, and what are his/her largest weaknesses? How can I force the character to face the latter in order to achieve the former?
If you’ve come up with the plot first and are searching for a character second, then think about what’s unique about the plot — what kind of unique skills, strengths, weaknesses can the plot test in someone? What kind of person would be pushed to the breaking point (or beyond) in this kind of plot?
At the end of the day, however you get to the place of having a character and plot, you should make sure the they’re integral to each other in a way that can’t be interchangeable. The plot should unfold because of who your character is and the actions they take (because remember, your main character should be making active decisions). And it’s these actions and their consequences that are going to push the character to grow and change.
This is a question I come back around to at all stages of constructing a story — the initial daydream stage, drafting, revising, and all point in between: why does it have to be *this* character in *this* story? What about this character is unique that makes this story unfold the way it does? And what about this story unfolding has a unique affect on my character?
*I also think they’re a hybrid of this where you have an idea of character and an idea of story and you build them both together.
** Yes, I’m mixing media here and I’m okay with that