The Development of Character
The superficial or flat character is flat, in a recognizable stereotype - the hero or the villain.

A round character has deeper qualities that makes them more complex and individual. Some writers use photographs as writing prompts to enrich their character's image.
Gilman was a clerk. He lived by rote. In spite of his outdoor body and his carbine in its slot under his stirrup- strap he was the kind of man who, if he had lived in a city, would have climbed on a high stool every day and put his figures in a ledger. There was no distinction of any kind about him, not in the way he rode. . .
40 Character_Development
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General Characteristics
Preferably, the main character in a story is dynamic. He or she is changed by the action in the story.
Authorial comment is the least effective method of characterization, but economical and can be vivid by using
sharp details.
from Duel in the Sun (Movie)
Authorial Interpretation
Narrators may tell their view of the character. Through exposition, they tell the reader what to believe: background, motives, values, virtues, or weaknesses. This can give the reader insights that might escape the characters themselves. We may tell our readers how to feel about the character and create contradictions to increase depth. This is the simplest way to describe a character.
Ways to Know a Character

Looks Other's Opinions Actions/Inactions Other's Reactions
Dress Conversation Thoughts/Feelings Narrator Exposition
Major Character Attributes

Believable Consistent Complex Memorable Shows Change