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 Saga
The Metal Gear series is notorious for breaking the fourth wall, sometimes 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 and gravure idol posters also appear in all of the Solid series games.

Metal Gear

 * After the player learns that Big Boss is the commander of Outer Heaven, Big Boss calls and orders the player to abort the mission and turn off the game console. He also sends misinformation in an attempt to confuse and kill Snake.

Metal Gear 2: Solid Snake

 * After the player reaches Tower Building, Campbell calls and tells Snake he's changing 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 instruction manual to decipher radio frequencies.

Metal Gear Solid and The Twin Snakes

 * The player must find Meryl's frequency using a screenshot on the back of the games packaging. Although the game doesn't say it, the Essential Collection version has her codec frequency in the manual along with on the back of the collectors box, due to the Kojima Productions logo put on the back of the game case instead.
 * 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.
 * During the battle with Psycho Mantis:
 * Psycho Mantis reads the player's Playstation memory card. If you have corresponding Konami-based savefiles, Psycho Mantis will comment on them. (eg. "I see you like Castlevania!") Mantis also does this in Twin Snakes, although with Nintendo (such as Super Smash Bros. Melee) or Silicon Knights (such as Eternal Darkness: Sanity's Requiem) game saves instead.
 * Psycho Mantis tells the player he will move his controller using his mind, and to place it on the ground. 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.
 * The player must physically change the controller's port on the Playstation or Gamecube, to prevent Psycho Mantis from "reading their mind." If playing on a Playstation 3, reassigning the controller works just fine.
 * 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 Dual Shock controller, 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.
 * 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.

Metal Gear: Ghost Babel

 * Before fighting Pyro Bison, Bison will tell Snake the number of sentries 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 something good, Right/L2 for something bad.
 * 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, MGS VR Missions and Ghost Babel, and famously spouting gibberish. Also, at one point, the radar screen shows a Japanese woman instead of the typical radar screen.
 * Snake, after meeting Raiden in the Arsenal Gear's bowels, remarks that he has "Infinite ammo." while pointing to his bandanna. This is a nod to Metal Gear Solid's infinite ammo bandanna item that is acquired in the Meryl ending.
 * When Raiden throws off his dogtags at the end of the game, they display the information that player entered at the beginning of the game: often considered a metaphor for Raiden's refusal to remain a puppet and to start thinking for himself.

Metal Gear Solid 3: Snake Eater

 * Snake describes wearing the Raikov mask as "nostalgic." This is obviously because Raikov is based on Raiden, the main character 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 you'll have to rely on your instincts as a gamer instead.
 * When EVA tells Snake about Raikov, Raikov is shown turning to the camera and smiling, an obvious nod at his likeness to Raiden.
 * After Snake and EVA are caught trying to sabotage the Shagohod, The Sorrow will float in the background holding a chalkboard with a countdown on it, showing the player how long Snake has to escape the hangar.
 * If Snake dies or kills Ocelot, Sokolov, or EVA, you'll be given a Game Over for creating a Time Paradox, as all four characters appear in games further on in the storyline.
 * In the Snake vs. Monkey minigame, Solid Snake references Gabe Logan and Sam Fisher, of Syphon Filter and Splinter Cell fame, respectively. Campbell also mentions characters from Ape Escape, commenting that one of them is friends with Otacon.

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.
 * When you enter the Blast Furnace in Shadow Moses, Otacon will call you by codec and tell you to change game discs but then realizes that you are playing the game on a dual layered Blu-ray disc and therefore do not need to change discs like you did at the same area in Metal Gear Solid.
 * The entire Screaming Mantis fight is used to play on your memories of the Psycho Mantis fight. At one point, Mantis will change your 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 "it 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 attempt to perform his old tricks once again. However, due to the hardware, he's unable to read your memory card and, if you're using a SixAxis controller, he won't be able to make the controller rumble either, and Naomi is heard saying "No more massages for you" referencing another scene in MGS1. Frustrated, Mantis will storm off.
 * If you're using a DualShock 3 for the above sequence, Mantis will successfully move the controller and gleefully declare that "Vibration is back!"
 * If a player calls Rosemary after being killed and selecting the continue option, Snake will occasionally start talking about dreams of dying or getting killed that he's been having.
 * 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 you die during the fight against Liquid Ocelot or while in the microwave hallway (both in Act 5), the "Exit" option becomes "Exist". If you choose exist, Liquid will reply, "Brother! It's not over, not yet!", a recurring phrase throughout the series. Then the marker will return to "Continue" and "exist" will be "exit" again.
 * In a conversation with Meryl as she discribes the SOP system the mentions that, " Every thing was just a game to them," as the screen will show all of the Metal Gear games and then she says, "But now it's reality." as the list gets to Metal Gear Solid 4 and the Under Construction titles.