The term "breaking the fourth wall" is used in film, theatre, television, and literary works, originating from Bertolt Brecht's theory of "epic theatre" that he developed from (and in contrast to) Konstantin Stanislavski's drama theory. It refers to a character directly addressing an audience, or actively acknowledging (through breaking character or through dialogue) that the characters and action are not real -- in other words, the audience is made explicitly aware of the fact that they are viewing a work of fiction. Various artists have used this jarring effect to make a point, as it forces an audience to see the fiction in a new light and to watch it less passively. Bertolt Brecht was known for deliberately breaking the fourth wall to encourage his audience to think more critically about what they were watching, referred to as Verfremdungseffekt (often translated to "alienation effect"). The phenomenon is also known as "canon puncturing"*.

The sudden breaking of the fourth wall is often employed for comical effect, as a sort of visual non-sequitur; the unexpected breaking from normal conventions of narrative fiction can surprise the audience and create humour.

Such exploitation of an audience's familiarity with the conventions of fiction is a key element in many works defined as post-modern, which dismantle established rules of fiction. Works which break or directly refer to the fourth wall often utilize other post-modern devices such as meta-reference or breaking character.

A compromise to the concept often occurs in improvisational theater, in which the audience is asked to interact with the players to some extent, such as by voting on a resolution to a mystery. In that case, the audience members are treated as if they were witnesses to the action in the play, effectively becoming "actors" rather than being a true "fourth wall." This is a major tenet of Augusto Boal's Theatre of the Oppressed.

The fourth wall is sometimes included as part of the narrative, when a character discovers that they are part of a fiction and 'breaks the fourth wall' to make contact with their audience, as seen in films such as the Last Action Hero or The Purple Rose of Cairo. In these situations however, the 'fourth wall' that the character breaks remains part of the overall narrative and the wall between the real audience and the fiction remains intact. These sorts of stories do not actually break the fourth wall in the strictest sense, but are more properly referred to as metafiction, or fiction that refers to the conventions of fiction.

It is arguable that this technique was first employed in the modern sense, (i.e. not in which an actor merely makes a clarifying aside to the audience, or clever implied self-references are made, but rather when the fourth wall is demolished to the point that there no longer remains any significant division between performance and audience, with drama joining reality or the exact opposite depending on one's perspective), in the sensational 1921 premiere of Pirandello's play Sei Personaggi in Cerca d'Autore, wherein six members of the audience at the rehearsal of a play suddenly demand that their stories be told as part of the performance.