What element of a play serves as a boundary between the audience and performers?

The fourth wall is a critical concept in theatre that refers to an invisible barrier between the audience and the performers. It serves as a boundary that allows the audience to observe the story unfolding on stage while maintaining a sense of separation from the performers. This concept creates an immersive experience where the audience can engage emotionally and cognitively with the action without directly interacting with the actors.

In traditional theatre, the fourth wall enables actors to perform as if the audience does not exist, thereby preserving the illusion of the narrative. When this wall is "broken," it creates a different dynamic, allowing for direct engagement with the audience, which can be used for comedic effect or to involve the audience in the story.

The proscenium arch is a physical feature of many theatres, framing the stage and helping to create a defined space for performances, but it does not embody the concept of the boundary itself as the fourth wall does. Stage curtains and audience seating are also important elements in the theatre environment, involved in the practical aspects of staging a performance and accommodating spectators, but they do not directly represent the conceptual barrier between performers and the audience in the way the fourth wall does.