Flat characters are minor characters in a work of fiction who do not undergo substantial change or growth in the course of a story. Often, these characters serve no purpose other than to move the story along, so who they are as people is of no significance to the plot.
In some instances, they may be more parts of the setting than characters. For example, if two main characters are eating in a restaurant, their server—if portrayed at all—likely would be a flat character.
When people say a character is static, they're referring to the fact that a character doesn't change. Such characters usually are a type of flat character. A key difference often is that static characters might appear in more than one scene.
Perhaps your main character lives in a building with a doorman, and to develop your character, you show the kind of small talk she engages in with the doorman each time she enters or leaves. The doorman would be described as a static character because he never changes; he is there only to help show a small part of the main character's personality.
Round characters, unsurprisingly, are the opposite of flat characters. For readers, these are the characters you will put the most effort into following and understanding, and for writers, they are the most challenging to develop. Round characters are multidimensional, complex, nuanced, and often contradictory.
Round characters don't have to be the main characters in a story, but they serve an important enough role in the plot or a subplot that giving their personalities multiple layers is a necessity.
Just as round characters are the opposite of flat characters, dynamic characters are the opposite of static characters. Dynamic characters also are round characters who will undergo some kind of change in the course of the story.
Consider how the doorman used as an example of a static character could instead be a dynamic character if fleshed out a bit more. As the doorman gets to know the main character a little better, perhaps he discovers something unsavory about her character and must decide whether to act on that information. Regardless of what he chooses, his encounters with the main character are likely to change as his perception of her changes.
Stock characters are similar to static characters, but they often represent a particular stereotype. 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 purpose of a stock character is to move the story along by allowing the audience to already understand the character.
For example, the main character might be on the run from the mob and encounters a mob enforcer. While it's unoriginal to present the character as a stereotypical goon, it does move the story along quicker because the audience already is familiar with what that stock character represents.
Protagonists are the main characters in your fiction. They are round characters with whom readers sympathize. However, they are not always completely moral or likable.
It's important for protagonists to be relatable even if they are not likable. Readers need to believe protagonists and understand their choices.
Captain Ahab in Herman Melville's "Moby Dick" is a good example of a protagonist who is relatable yet unlikable. Ahab's stubbornness and obsession with the whale that bit off his leg negatively impact the rest of the characters in the story, yet readers can relate to the emotion of becoming obsessed to the point of detriment to themselves and those around them.
Antagonists often are known as the bad guy in works of fiction. They attempt to prevent protagonists from getting what they want or need.
An antagonist also should be a round character. Making an antagonist evil is not as interesting as making the character conflicted. Pure evil is very hard to believe in fiction since people are multifaceted and inspired by their situations and personal histories.
One of the most iconic antagonists in film history is Darth Vader from "Star Wars." At first, he seems to be little more than the embodiment of pure evil for reasons that aren't clear. As the story develops, his character also is developed and viewers learn how and why he became such a violent and intimidating figure.
Just as people in real life, characters in fiction need someone in whom they can confide. Confidantes work best when they are round characters dealing with their own conflicts and issues, but they serve the story as someone who can help the protagonist in his own conflict.
In Leo Tolstoy's "Anna Karenina," Anna often turns to her sister-in-law, Dolly, to discuss the problems that have arisen because of her relationship with Vronsky. Dolly is an interesting choice because she herself has been victimized by her own husband's infidelities, and as the novel begins, it is Anna who is serving as Dolly's confidante.
A foil is someone who possesses the opposite traits of a main character, often the protagonist. The purpose of the foil is to serve as a contrast, which can help bring out the main character's best traits.
For example, if you create a character who is known for being honest, that can be highlighted by creating a foil for the main character who is consistently dishonest and perhaps challenges the main character's own commitment to honesty.
Character Types in Fiction Writing
Creating Believable Characters
As a writer or reader of fiction, you'll encounter many types of characters: round characters, flat characters, stock characters, and protagonists, among others. All characters have their place, yet you may want to avoid or handle some of these character types delicately.
Also, don't 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.