Format dialogue correctly and add interiority to reveal a character's private thoughts and feelings.
Dialogue needs two mechanical rules: quotation marks around exactly what is said, and a new paragraph every time the speaker changes. Getting this right keeps a reader from ever losing track of who is talking.
Interiority is a character's private thoughts and feelings -- the part no one else in the story can hear. Adding a line of interiority next to dialogue lets a reader know what a character is really thinking, even when they say something different out loud.
Mastery looks like: They format dialogue correctly (quotation marks, new paragraph per speaker) and add interiority that reveals a character's private thoughts.
Common stumbles: Running multiple speakers together in one paragraph; writing interiority that just restates the dialogue instead of adding a private, possibly contradicting thought.
See it visually
Interactive, spoken-aloud tools that show this idea in motion.