When a character or narrator directly acknowledges the audience or the fact that they exist inside a story.
Breaking the fourth wall is the deliberate act of a character or narrator addressing the audience, acknowledging the medium they exist in, or otherwise shattering the illusion that the story is a self-contained world. It can be as subtle as a narrator's aside ("But that, dear reader, is another story") or as dramatic as a character turning to the camera and saying, "You did not think that was going to work, did you?" When done well, it creates intimacy, humor, or a provocative kind of discomfort. When done poorly, it just yanks the reader out of the story.
Breaking the fourth wall is a high-risk, high-reward move. It can forge an electric connection between your character and the reader, add layers of irony, or make a commentary on storytelling itself. But it also disrupts immersion, so you need to know exactly why you are doing it. As a writer, understanding when and how to break the wall gives you a tool that most writers are too cautious to use.
Vonnegut inserts himself into the novel, blurring the line between fiction and autobiography and making the reader constantly aware of the act of storytelling.
The novel begins by addressing "you, the reader" and makes the act of reading itself the central plot - a fourth wall that is broken from the first sentence.
The narrator constantly addresses the reader, warns them to stop reading, and comments on the story's events from outside the action.
The protagonist's asides to the camera evolve from a comedic device into an emotionally devastating tool when another character notices her doing it.
Every fourth-wall break should serve the story - humor, intimacy, thematic commentary, or disorientation. If you are just being clever, the reader can tell.
If your narrator addresses the reader in chapter one and then never does it again, it feels like a mistake. Establish the convention and commit to it.
Breaking the fourth wall during a tense or emotional moment can deflate it. Make sure your breaks enhance the moment rather than undercutting it (unless undercutting is the point).
Write a 500-word scene where a character is in the middle of a serious conversation. Halfway through, have the narrator interrupt to speak directly to the reader - maybe to clarify something, confess something, or argue with the story's direction. Then return to the conversation. Notice how the interruption changes the scene's energy.