Characters, like people, are flawed and broken. Unlike people, who hide it well and have a lot of other stuff going on, characters are “sculpted.” Even deeply-layered characters aren’t the contradiction that people are. We tend to be messy; characters are tidy messes. Everything we know about a character has to be of use.

If you establish Bonnie is a birder, then Bonnie better be putting that talent to use in the story.
Because I’ve had a great upbringing, my tendency is to make nice characters. I’ve been surrounded by nice broken people, so it makes sense my characters are nice. I have to resist that tendency. Characters should have flaws, often big ones, that they don’t hide well.
Imagine your characterscape as a chess game. Every piece has an opponent. Your own guys can’t always move the way you can. They have different motives and this may make them untrustworthy. Batman fights crime because his parents were murdered in front of him; Superman fights crime because his rural, moral upbringing says the strong should look out for the weak. Batman uses tactics that Superman would disapprove of; Batman thinks Superman is naïve.
Characters are motivated by what they want; what they want is based on what they believe; what they believe can be wrong.
As the grandmaster, you are creating characters who generate conflict with one another. In Angels, the boy is ignorant; the angel is wise. Their conflict is not remotely violent, but it is a mental tug-of-war. In Maniac, Hud’s family life is drastically different than Jack’s, so while Hud wants a little adventure, Jack wants to escape. This creates a widening gap between best friends.
In fiction, you rarely want equal partnerships in character or motivation. Think marriage. Motives – different; outlook-different; physicality-different; conflict-guaranteed.
The kinds of character:
Main Characters: You get in their heads, you know more about them than anyone else, you root for them. These characters will change throughout the story. Also called the Protagonist.
Secondary Characters: These characters serve a purpose. They may be in the entire story or just part of it. We care about them to a degree. They should not overwhelm the main character (it’s easy to let them steal the show, for me at least. As an actor I’m always the secondary character, so I love to write them. I just have to be careful or the story becomes his).
Incidental Characters: They fulfill an immediate purpose and then they’re gone.
Meaningless Characters: You shouldn’t have any of these.
The Bad Guy: The main character’s opposition. We may not get in their head (but, then again, we might), we typically don’t root for them, and this character doesn’t have to change. It’s important to note that the “bad guy” may not be a villain, they may just be oppositely motivated. The key here is that this character INSTIGATES THE MAIN CHARACTER’S CHANGE. Also called the Antagonist. In spiritual fiction, God is typically an antagonist because his power, mercy, love, kindness is naturally opposed to the main character’s fallen nature.
Normally, you want a tight cast of characters. Small families, small teams, and a small extended cast. Unless you’re writing epic fiction, in which character discipline is even more important.
Strongly differentiate your characters in motives, humor, intelligence, physicality, and language. Imagine Lord of the Rings with only human characters; it would be a nightmare to keep them all separate. That’s an epic; make non-epic fiction manageable.
Questions always welcome!