Flat characters are minor characters in a work of fiction who do not undergo substantial change or growth in the course of a story.
When people say that a character is static, they're referring to the fact that a character doesn't change. This is really another way of saying a flat character.
As a writer, your focus will be on developing round characters. For readers, these are the characters you will put the most effort into following and understanding. Round characters are multi-dimensional and complex. They are nuanced and often contradictory.
The opposite of static characters, dynamic characters are also round characters that will undergo some kind of change in the course of the story.
Many people think the term "stock characters" is another way to describe static characters, but not so. Stock characters are often stereotypical. They are difficult to pull off in fiction unless you are writing satire, and even then, there must be much thought behind including a stock character in your narrative. The point of the stock character is to move the story along by allowing the audience to already understand the character.
Protagonists are the main characters in your fiction. They are round characters which the reader often sympathizes with or roots for. However, they are not always completely moral or likable.
Protagonists should be complex and flawed. A big mistake that many beginning writers make is to worry too much about whether their protagonist is likable. Of greater importance is whether or not the protagonist is relatable. The reader needs to believe the character, and understand their choices.
The antagonist is essential to many works of literature and is usually known as "the bad guy." The antagonist is the person who is preventing the protagonist from getting what he wants or needs.
An antagonist should also be a round character. Making an antagonist evil is not as interesting as making him or her conflicted. Pure evil is very hard to believe in fiction since people are multi-faceted and inspired by their situations and backstories. Therefore, putting time into describing your antagonist and showing his or her struggles will make for a richer and more complex narrative. Just as a protagonist should not always be the good guy, an antagonist should not always be the bad guy.
Character Types in Fiction Writing
Creating Believable Characters
There are many types of characters that you may encounter as a reader or writer of fiction. You may have a round character, flat character, stock character, or protagonist. The list goes on. Learn more about the various types of characters, what they mean, and how to use the character or interpret the character type.
Please note that some of these character types you may want to avoid or handle delicately. Do not be discouraged if you receive critiques telling you that your character is flat. Instead, take it as a challenge, and see how emotionally complex and detailed you can make your characters.