fourth wall, in theatre, television, film, and other works of fiction, a convention that imagines a wall existing between actors and their audience. The wall is invisible to the audience, so viewers can see the performance, but opaque to the actors, blocking them from the audience. Thus, performers act as if the audience is not there, thereby resisting the temptation to address the audience directly or look directly into the camera, leaving the viewer to become a kind of voyeur, observing the narrative, resulting, it is believed, in a more realistic performance. The imaginary wall is part of the "suspension of disbelief" by the audience deemed critical to an appreciation and enjoyment of works of fiction.
The concept of the fourth wall is attributed to Denis Diderot, who believed that by ignoring the audience, performers could more closely imitate reality. In 1758 he wrote, “Imagine a huge wall across the front of the stage, separating you from the audience, and behave exactly as if the curtain had never risen.” The concept of the fourth wall became more well-known with the Naturalism of 19th-century theatre. In the 20th century, however, actors began to acknowledge the audience, which led to the term breaking the fourth wall. Oftentimes, performers spoke directly to viewers to provide commentary, to narrate their thoughts, or to acknowledge the very artifice of their production. Films such as Ferris Bueller’s Day Off (1986) and the series Fleabag (2016–19) were widely celebrated for their skillful use of the concept.