Fourth wall

The fourth wall is a term used to describe the imaginary wall between the audience and the stage, or in this case the gamer and the video game. The term derives from a movie or television set having three walls; the fourth wall is the camera. In effect, it is the separation of the real and the imagined. The origin of the term the fourth wall was first used in theater, where actors would acknowledge the presence of the audience through actions or speech. In the video game world, the fourth wall is often addressed in a humorous manner. In the early days of gaming, such things as putting certain information not available in-game in instruction manuals or on box art were rudimentary forms of software piracy prevention.

Origin
The term "fourth wall" stems from the absence of a fourth wall on a three-walled set where the audience is viewing the production. The audience is supposed to assume there is a "fourth wall" present, even though it physically is not there. This is widely noticeable on various television programs, such as situational comedies, but the term originated in theater, where conventional three-walled stage sets provide a more obvious "fourth wall."

The meaning of the term "fourth wall" has been adapted to refer to the boundary between the fiction and the audience. "Fourth wall" is part of the suspension of disbelief between a fictional work and an audience. The audience will usually passively accept the presence of the fourth wall without giving it any direct thought, allowing them to enjoy the fiction as if they were observing real events. It is the invisible barrier between realities.

The presence of a fourth wall is one of the best established conventions of fiction and as such has led some artists to draw direct attention to it for dramatic effect. This is known as "breaking the fourth wall."

Fourth wall references in the Metal Gear series
The Metal Gear series is notorious for breaking the fourth wall. Sometimes the fourth wall is broken many times in a single game. In fact, one could argue that Metal Gear is the most famous fourth wall breaking game series, given the popularity and infamy of the Psycho Mantis boss battle from Metal Gear Solid.

Common Elements

 * Snake's support team will describe actions using terminology such as "circle," "square," and "action button," referring to the game controller in use.
 * Gaming magazines featuring Metal Gear games can often be seen.

Metal Gear

 * Just before the player learns that Big Boss is the commander of Outer Heaven, Big Boss will call and order the player to abort the mission and turn off the game console.

Metal Gear 2: Solid Snake

 * After the player reaches the Tower Building, Campbell calls and tells Snake he's changing radio frequency. In the MSX2 version, he says that the frequency is on the rear side of the game's package. In the Subsistence version, it is supposedly in the game's manual.
 * The player must use a tap code found in the game's manual to decipher radio frequencies.

Metal Gear Solid/The Twin Snakes

 * The player must find Meryl's frequency using a screenshot on the back of the game's packaging. In The Essential Collection version of Metal Gear Solid, her codec frequency is in the manual. It's also featured on the back of the collector's box, due to the Kojima Productions logo put on the back of the game case. In the PlayStation 3 version (via the PlayStation Network), it is in the game's digital manual.
 * Master Miller warns the player to use the bathroom frequently as to not miss one of the game's cutscenes, and to not play while tired, after eating or after a bath.
 * During the battle with Psycho Mantis:
 * Psycho Mantis reads the player's memory card. If the player has corresponding Konami-based save files, Psycho Mantis will comment on them (eg. "you like Castlevania don't you?"). Mantis also does this in The Twin Snakes, although with Nintendo game saves instead (such as Super Smash Bros. Melee or Eternal Darkness: Sanity's Requiem).
 * Psycho Mantis tells the player he will move his controller using his mind, and to place it on the ground. In The Twin Snakes the camera turns to Snake, who nods approvingly, as to say "go ahead, it's safe."
 * Using his blackout technique, Psycho Mantis will change the player's screen to a black screen reading "HIDEO" in the top corner, as to replicate most standard television set's VIDEO setting.
 * Campbell tells Snake that he must physically change the controller's port on the PlayStation or GameCube, to prevent Psycho Mantis from "reading their mind." If played on a PlayStation 3, reassigning the controller works just fine.
 * In The Twin Snakes, Mantis will also cause "sanity effects" during the battle by making the overhead camera tilt at an angle and cracking the screen if the player manages to make Meryl shoot at the screen.
 * When being tortured by Revolver Ocelot, he remarks that if the player uses a turbo controller's "autofire", he'll know and automatically kill Snake. He also mentions that there "are no continues, my friend." Obviously, this is the only part of the game where if Snake dies, the player can't continue.
 * If playing with a DualShock controller (or, in have the vibration on in The Twin Snakes), Naomi will tell the player "to put the controller against (their) arm." She will then activate the controller's vibration functions, simulating a shiatsu massage. This occurs after the player has resisted two rounds of torture and is ready to escape.
 * While fighting the Hind D, Naomi and Campbell will advise Snake via codec to use the television's stereo speakers to follow the direction of the Hind. However, if the audio is set to monaural, they will instead express their pity towards Snake for not owning a stereo TV set.
 * In The Twin Snakes, the screen will crack if Snake is shot while in first person view, a gameplay element carried over from Metal Gear Solid 2: Sons of Liberty.

Metal Gear: Ghost Babel

 * Before fighting Pyro Bison, Bison will tell Snake the number of sentries or bosses the player has killed up to that point.

Metal Gear Solid 2: Sons of Liberty

 * The player can press the shoulder buttons on the controller to hear what either Snake or Raiden are "thinking" during codec conversations if they are not currently saying something; Left/L1 for positive thoughts, Right/R2 for negative thoughts.
 * Whenever Snake or Raiden is killed in first person view, the screen cracks.
 * Snake tells Raiden that he needs to have the controller's vibration function on to use the AP Sensor, and Olga tell him the same when speaking about the Cell Phone.
 * The entire Arsenal Gear sequence, after the virus is implanted onto GW, is a fourth wall break. Colonel Campbell and Rosemary make cryptic messages, urging the player to turn off the console, saying lines from previous games - Metal Gear, Metal Gear 2, Metal Gear Solid, VR Missions and Ghost Babel, and famously spouting gibberish (i.e. "I need scissors! 61!"). At one point, he even specifically calls Raiden's situation as a "role-playing game."
 * Snake, after meeting Raiden in the Arsenal Gear's bowels, remarks that he has "infinite ammo" while pointing to his bandana. This is a nod to Metal Gear Solid 's bandana. Once acquired, it gives the player infinite ammo for all weapons.
 * Just before Raiden throws away his dogtags at the end of the game, they display the information that the player entered at the beginning of the Plant chapter. This is often considered a metaphor for Raiden's refusal to remain a puppet and to start thinking for himself.
 * In actuality, the whole message of Metal Gear Solid 2 's story breaks the fourth wall. Raiden's role in his "scripted" mission parallels the player's role in respect to the game. Snake directly refers to the fact that Raiden's only experience is in VR as "war as a video game", during which scenes from Metal Gear Solid: Special Missions are played. In addition, scenes from the game's menu and images such as that of the "Fission Mailed" screen are shown during the Colonel's explanation of the S3 system.
 * In the Snake Tales story External Gazer, Solidus talks to Snake about the Koppelthorn Engine and how it can let people control parallel universes, and vice versa. He asks Snake, "Have you ever felt like your body wasn't your own, like you took an unexpected action after the fact? Or you were able to do something you didn't think possible? That's because of interference from other universes. Perhaps the reason you were able to topple Shadow Moses is because of the helping hand from someone in another universe." This is a nod to the players of Metal Gear Solid, since they are the ones who control Snake. This could also be a reference to Raiden and how he partook in the Shadow Moses Island VR missions.

Metal Gear Solid 3: Snake Eater

 * Naked Snake describes wearing the Raikov mask as "nostalgic." This is obviously because Raikov is based on Raiden, the protagonist of Metal Gear Solid 2.
 * During a conversation with The Boss, Snake mentions that he can't smell. Actually, it is the player that can not use their sense of smell to help play the game. The Boss then says that the player would "just have to rely on your instincts as a gamer."
 * In the basement of the Granini Gorky lab (heading towards the Granin cutscene, not the prison), there are video game magazines scattered on the floor.
 * When EVA tells Snake about Raikov, Raikov is shown turning to the camera and smiling, an obvious nod at his likeness to Raiden.
 * If Snake dies or kills Ocelot, Sokolov, or EVA, a Time Paradox screen is shown, as all four characters appear in games further on in the storyline.
 * In the Codec call that takes place just before the player plays the Snake vs. Monkey minigame, Solid Snake references stealth genre heroes Gabe Logan and Sam Fisher, of Syphon Filter and Splinter Cell fame, respectively. Snake also mentions Spike and Jimmy, two of the protagonists from the Ape Escape series. Campbell mentions other characters from Ape Escape, commenting that one of them is a friend of Otacon's.
 * Para-Medic uses some of the movies she mentions as real world metaphors for the players not to play too long or to take the game too seriously.

Metal Gear Solid Mobile

 * When the AI starts crashing due to Otacon's hacking, Commander tells the player to turn off their cellphone.

Metal Gear Solid 4: Guns of the Patriots

 * Old Snake uses a DualShock 3 controller to operate the Metal Gear Mk. II or III. This is also present in the game's instruction manual, where an artwork of Snake controlling the miniature Metal Gear with the same controller, along with instructions regarding the control scheme beneath the artwork, are all shown.
 * When Snake enters the Blast Furnace in Shadow Moses, Otacon contacts him by codec and tells him to change game discs (this is where the disc change occurs in Metal Gear Solid). He quickly, however, realizes that a "Dual-layered" disc eliminates the need to do so, complementing the PlayStation 3 hardware.
 * The entire Screaming Mantis fight is used to play on the player's memory of the Psycho Mantis fight. At one point, Mantis will change the screen to read "HIDEO 2", but if the player changes the controller port, Snake will still be unable to move and Otacon will tell him that tactic "won't work this time." Colonel Campbell suggests this technique if the player contacts Rosemary on the codec, and Rosemary will point out the flaw in his logic. Another plan suggested by Campbell is shooting off the leather straps from the Psycho Mantis bust, another tactic from the earlier game, but Snake will point out that there is no bust in this area.
 * After defeating Screaming Mantis, Psycho Mantis will appear and repeat his psychic and telekinesis performance from Metal Gear Solid. However, due to the hardware, he is unable to read the memory card, and his failed attempt to vibrate the SIXAXIS controller prompts an annoyed outburst (Naomi is heard saying "No more massages for you" referencing another scene in Metal Gear Solid).
 * If the player uses a DualShock 3 controller for the above sequence, Mantis will successfully move the controller and gleefully declare that "Vibration is back!"
 * If the player calls Rosemary after being killed and selecting the continue option, Snake will occasionally start talking about dreams of dying or that he's "died recently."
 * In a codec conversation Otacon asks if Vamp is immortal, to which Snake replies, "Not a chance. This is the real world, not some fantasy game."
 * If Snake dies during the fight against Liquid Ocelot or while in the microwave hallway (both in Act 5), the "Exit" option becomes "Exist." If "Exist" is chosen, Liquid will reply, "Snake! It's not over yet!", a recurring phrase throughout the series.
 * In a conversation with Naomi as she describes the SOP system, she mentions that the PMC soldier's battle experience "was just a game to them." During this conversation, the covers of all the previous games are shown flashing across the screen in sequence, with two blank squares saying "Under Construction", which is most likely talking about Metal Gear Solid: Rising and Metal Gear Solid: Peace Walker.
 * When Snake enters the house after following the resistance member in Act 3, the player can see posters referencing previous games such as Metal Gear Solid, Metal Gear Solid 3, and Metal Gear Solid: Portable Ops. These posters are artwork done by Noriyoshi Ohrai.

Super Smash Bros. Brawl

 * During a Codec conversation about Pit, Otacon mentions how Snake knows all about "Game Overs."
 * During the Subspace Emissary storyline, Snake says "Kept ya waiting, huh?", both breaking the cinematics' tradition of refraining from spoken dialogue and directly addressing the players.