The fourth wall is the imaginary "wall" at the front of the stage in a traditional three-walled box set in a proscenium theatre, through which the audience sees the action in the world of the play. The concept is usually attributed to the philosopher, critic and dramatist Denis Diderot. The term itself was used by Molière. The fourth wall illusion is often associated with naturalist theatre of the mid 19th-century, and especially with the innovations of the French director André Antoine.
The restrictions of the fourth wall were challenged in 20th-century theatre. Speaking directly to, otherwise acknowledging or doing something to the audience through this imaginary wall – or, in film, television, and video games, through a camera – is known as "breaking the fourth wall". As it is a penetration of a boundary normally set up or assumed by works of fiction, this is considered a metafictional technique. In literature and video games, it occurs when a character acknowledges the reader or player.
Breaking the fourth wall should not be confused with the aside or the soliloquy, dramatic devices often used by playwrights where characters on stage are delivering inner monologues, giving the audience insight into their thoughts.
Convention of modern theatre
The presence of the fourth wall is an established convention of modern realistic theatre, which has led some artists to draw direct attention to it for dramatic or comedic effect when a boundary is "broken", for example by an actor onstage speaking to the audience directly. It is common in pantomime and children's theatre where, for example, a character might ask the children for help, as when Peter Pan appeals to the audience to clap for Tinkerbell. One play that uses the fourth wall extensively for comedic effect is The Complete Works of William Shakespeare (Abridged).
The acceptance of the transparency of the fourth wall is part of the suspension of disbelief between a fictional work and an audience, allowing them to enjoy the fiction as if they were observing real events. Critic Vincent Canby described it in 1987 as "that invisible scrim that forever separates the audience from the stage".
Lowry ... stood across the road from his subjects and observed. Often enough there are a number of individuals in a crowd peering back at him. They invite us momentarily into their world, like characters on a stage sometimes do, breaking the fourth-wall illusion.
McKellen justifies this application of the theatre term to Lowry's art by explaining that "Lowry’s mid-air viewpoint is like a view from the dress circle", looking down as if to a stage. And, McKellen argues, Lowry "often marks the limits of the street scene with curbstones or a pavement that feel like the edge of the stage where the footlights illuminate the action."
In books the metaphor of breaking the fourth wall has been applied to works that "insert real-life authors directly into the fictional construct as narrators or characters." Breakfast of Champions, by Kurt Vonnegut is a novel that has been described as using this technique in a way that is similar to the fourth wall breaking theatre of Bertolt Brecht. The metaphor of the fourth wall has also been applied by literary critic David Barnett to Bored of the Rings, The Harvard Lampoon's parody of The Lord of the Rings when a character breaks the conventions of storytelling and refers to the text itself by saying, "it was going to be a long epic". This, in Barnett's view, "breaks the 'fourth wall'".
Comedy films by Mel Brooks, Monty Python, and Zucker, Abrahams and Zucker frequently broke the fourth wall with characters speaking or reacting to the audience so frequently that with these films, "the fourth wall is so flimsy and so frequently shattered that it might as well not exist", according to the A.V. Club. Woody Allen broke the fourth wall several times in his movie Annie Hall, as he explained, "because I felt many of the people in the audience had the same feelings and the same problems. I wanted to talk to them directly and confront them." The John Hughes movie, Ferris Bueller's Day Off is also another well known fourth-wall breaking movie. Ferris Bueller, played by Matthew Broderick, often turns to the camera and breaks character to tell his thought process or explain his reasoning.
On television, breaking the fourth wall is rare, though it has been done throughout the history of the medium. George Burns did it numerous times on The George Burns and Gracie Allen Show he starred in with his real-life wife Gracie Allen from 1950 to 1958. It's Garry Shandling's Show and Mrs. Brown's Boys both have their title character walking between sets mid-scene, and the latter occasionally shows characters retaking fluffed lines.
Another television character who regularly breaks the fourth wall is Francis Urquhart in the British TV drama series House of Cards, To Play the King and The Final Cut. Urquhart addresses the audience several times during each episode, giving the viewer comments on his own actions on the show. The same technique is also used in the American adaptation of House of Cards by main character Frank Underwood.
The convention of breaking the fourth wall is often seen on mockumentary sitcoms, including The Office. Mockumentary shows which break the fourth wall poke fun at the documentary genre with the intention of increasing the satiric tone of the show. Characters in The Office directly speak to the audience during interview sequences. Characters are removed from the rest of the group to speak and reflect on their experiences. When this occurs, the rules of impersonal documentary are shattered. The person behind the camera, the interviewer, is also referenced when the characters gaze and speak straight to the camera. The interviewer, however, is only indirectly spoken to and remains hidden. The technique of breaking the fourth wall which is seen in shows with complex genres, serves to heighten the comedic tone of the show while also proving that the camera itself is far from a passive onlooker. 
In literature, writers often break the fourth wall of a story by having their narrator or characters address the reader either in footnotes or other literary devices, such as using free indirect speech, thus having the narrator or the novel recognize itself as novel or narrator.
The term "fifth wall" is often used by analogy with the "fourth wall" for a metaphorical barrier in engagement with a medium. It has been used as an extension of the fourth wall concept to refer to the "invisible wall between critics or readers and theatre practitioners." This conception led to a series of workshops at the Globe Theatre in 2004 designed to help break the fifth wall. The term has also been used to refer to "that semi-porous membrane that stands between individual audience members during a shared experience." In media, the television set has been described metaphorically as a fifth wall because of how it allows a person to see beyond the traditional four walls of a room. In shadow theatre the term "fifth wall" has been used to describe the screen onto which images are projected.
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