I believe in courtesy, even if I forget it sometimes. I also believe that writers have obligations to three life forms:
1) Ourselves- first and foremost, we write the stories that we will enjoy writing, that we will slave over because we enjoy writing that line of action or inserting a line of humor. This is a courtesy to yourself because most writers need day jobs, so if they're investing time and possibly money into a hobby, they better enjoy it.
2) Readers- You owe your fans and your personal readers a good story. Period. Not because of some weird obligation, but because you were once
a fan of something, and you know what it's like when you hit a twist ending or your author essentially says "I just want your money haha" when they write a horrible sequel or prequel. Do unto readers as other writers unto you.
3) Characters- They may only exist in your notebooks, but they're the ones that your readers fall in love with. Every story needs a character, specifically a protagonist and a conflict. Your protagonists matter the most in how they're shown as they grow through a story.
The novel functions as a democratic medium because it shows most of a character's three-dimensional layers. If you write many novels, you have to add more to that person's character IF you are writing a serious story. With more fun novels, like the Pippi Longstocking books, you can get away with simply having the same character in different circumstances. The bandits from the first Pippi Longstocking book differ greatly from Jim and Buck in the third book, while similar old ladies have different reaction to this peculiar carrot-top.
Another good facet that some series writers do is add a layer of character to their mean, unsympathetic characters. This sometimes fails if we have hated this antagonist for a rough decade and suddenly we see their perspective, like Severus Snape, at the tail end, but it's an effective tactic to open up a fictional world when done right. Edmund from the Narnia books is such an example; he's horrid in book one, but he matures to the point that he and his sister Lucy can enter Narnia without their older siblings.
An absolute no-no, however: no matter what you do, you CANNOT make sympathetic characters into monstrous or even unlikable villains, ESPECIALLY if their actions are out of character. While C.S. Lewis did well with Edmund and redeemed him, he lost all sympathy for Susan of little faith. In the second book, Aslan forgives her for being sensible and
thinking that he no longer exists, but The Last Battle shunts her out of the picture. Her siblings say she has gotten interested in "nylons" and doesn't believe in Narnia anymore; we never hear her side of the story.
C.S. Lewis by then had nurtured a fanbase, and they all called out his mistake, but he stayed by his decision despite his reputation as a kind, religious man. Whether it be prejudice (Edmund and Eustace were both horrible in the Narnia books when they first appeared but soon became heroes) , a comment on flapper girls and rampant materialism, or a reenacted fall from grace, Lewis betrayed Susan and the democracy of his books by turning her "villainous," or at the very least unsympathetic. Even worse, he didn't show us the story from Susan's perspective, or at least tell us what happened. This came back to haunt American and British literature, leading to gory works like "The Problem of Susan" by Neil Gaiman and modern complications for the directors doing the films.
Remember Susan Pevensie, folks. Remember her well.