For an author, their characters are their children, born in their heads, and loving crafted on the page. The trick comes in creating characters that aren’t the author, good or bad. I’m not just taking Mary Sues here, wish-fulfillment stand-ins for the author, I’m taking every character in the story ultimately lining perfectly up with the author’s point of view, thereby being intellectual extensions, even if they aren’t literal extension.
The Left Behind Series is a good example of this, where in the end, there’re no characters who really end up arguing against the author’s views. Sure there’s Chloe , who puts up a meager argument, but she ends up converting and falling right in line. Even the bad guys are bad guys simply because they have to be bad according the author. This is a big reason the Left Behind Series sucks.
Now I’m not saying a story can’t have a moral or a point of view, what I’m saying is, the characters shouldn’t know they’re in a story with that point of view.
Let’s a take a sillier issue because I don’t want to start a huge debate about other things, and that you want to write a story about why eating pizza is bad. A badly written story would be one in which all the people who don’t eat pizza are automatically good and know they are good simply because of it, and all the people who do are automatically bad and know they are bad because of it. A better written story would be where eating pizza is a symptom of an underlying issue that makes a person bad, they eat pizza because they don’t care about their health. A good person is one who doesn’t eat pizza because they want to eat healthy because they care about their health.
I know there is healthy pizza but work with me here, because you can copy and paste any issue you want into the preceding paragraph and get my underlying point. Characters should be in a world that exists outside of the author’s bias, a world where characters think on their own and do not see themselves as evil, because cartoon villains do not exist. There should be a reason bad people are bad people, one that is ultimately human(mental illness counts here). Good characters should be held to same standard, being good for a reason, not simply because automatically are.
Even I have trouble doing this in my work and some characters will come off as 2d, but the ones who do are the ones who are there for a page or two and vanish. There’s no time to develop them and I’m not wasting words on throwaway characters. But if a character is there for a while, I’ll try to make them as full as possible.
The deeper your characters are, the deeper and more believable your world becomes.