Box set (theatre) in the context of Dion Boucicault

The fourth wall is a performance convention in which an invisible, imaginary wall separates the actors from the audience. While the audience can see through this "wall", the convention assumes that the actors behave as if they cannot. From the 16th century onward, the rise of illusionism in staging practices—culminating in the realism and naturalism of the theatre of the 19th–century—led to the development of the fourth wall concept.

The metaphor relates to the mise-en-scène behind a proscenium arch. When a scene is set indoors and three of the room's walls are depicted onstage—forming what is known as a box set—the "fourth" wall lies along the line (technically called the proscenium) dividing the stage from the auditorium, effectively where the audience sits. However, the fourth wall is a theatrical convention, not a feature of set design. Actors ignore the audience, focus entirely on the fictional world of the play, and maintain immersion in a state that theatre practitioner Konstantin Stanislavski called "public solitude" —the ability to behave privately while being observed, or to be "alone in public." This convention applies regardless of the physical set, theatre building, or actors' proximity to the audience. In practice, actors often respond subtly to audience reactions, adjusting timing—particularly for comedic moments—to ensure lines are heard clearly despite laughter.