A writer’s major characters and major supporting characters are called round characters, as they are well-rounded people in the context of the story. They are complex, emotional, and have many layers of different feelings. And each has strengths, weaknesses, and qualities that distinguish them from the rest of the characters. The lead character (and sometimes one or two other major characters) is given opportunity to grow—to improve upon himself and flourish by the end of the story.
Another major, well-rounded character is the villain, or antagonist. The antagonist is equal in importance to the protagonist, and is a confrontational character motivated to cheat and harm the main character. Complex and emotional people, despite their dishonesty, questionable morals, and cowardice—three ingredients to every villain’s downfall—well-rounded antagonists seem to leap from the pages.
An antagonist can be an element instead of a person. I wrote stories about a ridge and its supernatural power over the populace. That kind of antagonist is an internal obstacle not an external one, like the Barrens and the shape-shifting creature in Stephen King’s It. These types of antagonists must have a heart that can be destroyed in order for the protagonist to become a hero.
Unlike well-rounded characters, flat characters are minor characters that authors use as props in a story. Think of waiters, cab drivers, pizza delivery persons, etc. They are not well developed, have no depth or scope, and they never grow and evolve during the story. Instead, they stay set in their ways. Theiir roles in a story are short and specific: to advance the plot, or provide a necessary setting. They may even contribute in conversation, usually to pass along important information. That’s all.