The protagonist is the main character whose journey drives the plot, while the antagonist is the character or force opposing them. Antagonists need not be villains; an antagonist is simply whatever stands in the way of the protagonist's desire. Around these two poles, writers populate their stories with a range of figures: round characters are complex and fully realized, while flat characters are one-dimensional and limited. A static character does not change over the course of the story, whereas a dynamic character undergoes significant transformation. A foil character exists specifically to contrast with another, highlighting traits by opposition.
Characterization is the way a writer reveals character, and it can be direct—explicitly stating a character's traits—or indirect, allowing traits to emerge through actions, dialogue, thoughts, and reactions. Most skilled writers favor indirect characterization because it lets readers participate in the discovery. Interiority, the rendering of a character's inner thoughts and emotional life on the page, is one of the deepest tools available, and showing emotion through action—conveying feelings via gestures, behavior, or physical reactions—is often more powerful than naming an emotion outright.
Character arcs track the transformation a character undergoes through a story. In a positive change arc, the character grows for the better; in a negative change arc, the character degenerates or fails; in a flat arc, the character stays essentially the same and instead changes the world around them. Certain archetypes recur across literature: stock characters like the mentor are common and recognizable, while a Mary Sue is so idealized as to lack believable flaws. An antihero lacks conventional heroic qualities, and a tragic hero is brought down by their own flaws. The tragic hero's fall, when skillfully rendered, produces the catharsis that has defined tragedy since Aristotle.