According to a 1998 The PlayStation magazine interview with director/producer/writer Hideo Kojima & designer/artist Yoji Shinkawa, they commented about designing the character Psycho Mantis, specifically how elements such as him wearing a gas mask because he had "no face" were "part of expanding and developing the story", according to Shinkawa. Kojima clarified that while he didn't know why Shinkawa made those choices, he was not against them being made, stating that "unexpected things like this are part of the fun".

Early concept art of Psycho Mantis by Shinkawa includes notes that read: "His body is like a preying mantis, spindly and emaciated. He wears a mask to hide his ugly face." and "Is this Psycho’s secret real face?"

During the fight with Psycho Mantis, the player is intended to swap their controller to the second port to prevent Psycho Mantis from reading their mind. However, if you call Campbell after losing to Psycho Mantis enough times, Campbell and Naomi will reveal that the statues in the room are based on Mantis himself, and attacking the statues will distract him long enough to be attacked. This alternate strategy was implemented in case the second controller port of the PlayStation is damaged, although this strategy was not included in Metal Gear Solid: The Twin Snakes on the GameCube.

According to a 1998 The Playstation magazine interview with director/producer/writer Hideo Kojima & designer/artist Yoji Shinkawa, Kojima originally had no intentions of adding the cyborg ninja character design for Gray Fox to the game, but Shinkawa drew that idea "because it's cool". At first, Shinkawa thought Gray Fox should have looked like a normal enemy soldier. He had several different patterns in mind, and thought to try drawing a cyborg-ish and ninja-looking guy, of which that design came out as a result.

Despite Shinkawa wanting his design to become the main character in his own game, it didn't. Raiden, however, became a cyborg ninja in Metal Gear Solid 4: Guns of the Patriots, and later became the main character of his own game in Metal Gear Rising: Revengeance.

In the English localization of the game, when Ocelot and Liquid Snake discuss their demands to the president, Ocelot states their demands as including "1 billion dollars" for both the original PlayStation/PC release and the GameCube remake. In the original Japanese release of the PlayStation/PC version, he states it as "50 million dollars". However, for the Japanese release of the GameCube remake, there is no dub of the game's voice lines in Japanese, while the subtitles translated to Japanese still say "1 billion dollars". Whether or not "1 billion dollars" or "50 million dollars" is a mistake in the game's script or was changed for future releases of the game is unknown.

After defeating Psycho Mantis and listening to his final words in the Japanese version of the game, Mantis will say "it feels nostalgic." In the English version, the dialogue is changed to "it feels kind of nice," although this was then changed back into "it feels nostalgic" in the remake Metal Gear Solid: The Twin Snakes.

According to Hideo Kojima, the "nostalgia" that Mantis refers to is from him sensing his own mother's feelings during childbirth, where she used the last of her strength to help him survive.

The scene in which Sniper Wolf Spoiler:shoots down Meryl and continues to repeatedly shoot her in order to bait Snake out from cover is based on the same scene from the 1987 Stanley Kubrick film Full Metal Jacket.

After the first fight with Sniper Wolf, if the player chooses to shoot and kill the rats on the platform above where Sniper Wolf originally was, they will receive codec calls from several of the teammates expressing their distaste towards Snake's actions.

Before the torture sequence, if the player had no saved often during play, Ocelot will comment on it by remarking that they would have to "travel down that long road again" and telling him to just "give up now" in order to avoid getting killed.

After the first torture session with Ocelot, if that player runs around the cell, perform attacks inside the cell, or knocks on the cell walls, the guard will yell at Snake to be quiet.
Hiding under the bed while he is still watching you will also make him comment at Snake, saying "What the hell are you doing?", and using the Ketchup while still in his view will make him say "What are you playing around with that Ketchup for?"

While in the cell after the first torture session, if the player calls Naomi on the codec, she will eventually tell Snake to "place the controller against your arm" and makes the actual PlayStation controller vibrate to sooth the "pain in the arm", referencing the player having to mash the O button to keep Snake alive while he was being tortured.

The psychic powers of Psycho mantis were inspired by the 1978 film The Fury. Director Hideo Kojima even told the motion designer to see the movie so he can see a scene where a young man flies with these powers.

After the first time Meryl gets possessed by Psycho Mantis just outside of the Commander's Room, using the first-person perspective will reveal it has been temporarily changed to Meryl's point of view, instead of Solid Snake. Spoiler:This is a possible foreshadowing to the Psycho Mantis fight, where trying to use the first person perspective in it will actually show Psycho Mantis's point of view instead of Solid Snake's.

Metal Gear Solid borrows a good number of gameplay/plot elements from the previous game, Metal Gear 2: Solid Snake. The similarities are numerous, but the most notable ones are:
• The idea of looking on the back of the game's packaging/cd case for a hidden codec frequency. Both contact a female contact that help Snake out on his mission and both are even the same frequency, Spoiler:140.15. Both also get romantically involved.
• The idea of having to change the temperature of an object to turn it into a key.
• The idea of an anonymous source of help contacting Snake, Spoiler:both of which end up being Gray Fox.
• Snake has to run up a long flight of stairs while being chased by a number of soldiers in both games.
• The idea of a 4-man ambush inside of an elevator. The Metal Gear Solid version differs in that the four soldiers that attack are wearing stealth camouflage, which was also taken from Metal Gear 2 as there was a soldier who attacks Snake while wearing Stealth Camouflage.
• A battle with a Hind-D using Stinger Missiles.
• Snake has to battle an operational Metal Gear, and after destroying them he then has to fight the pilot of said Metal Gear in hand-to-hand, as his gear is taken away from him.

If the player hits Meryl, she will retaliate with a hard slap, knocking off health. Meryl can end up killing Snake through this, and when she does the Game Over screen will have her voice saying "This can't be!"
If the player does this in the caves portion of the game and the dogs are around, she will call for the dogs to attack Snake. If the player immediately hides in the cardboard box after attacking Meryl, the dogs will instead pee on the box Snake is hiding inside of.

After the cut-scene inside the bathroom with Meryl, if the player goes into 1st person mode and stares at her, she will ask Snake to "stop staring at her like that." If the player continues to stare at her in 1st person mode, her skin will slowly turn redder from blushing.

Shortly after defeating Grey Fox and meeting up with Hal, the player will have to find Meryl, who is currently disguised as a Genome Soldier. When the player finds her, she will run off into the women's bathroom and change out of her disguise.
If the player follows her quickly enough, the next cut-scene will show her without any pants on, as she didn't have enough time to change properly.
However, if the player goes into the bathroom before seeing Meryl and calls Mei Ling on the codec, Mei Ling will chastise Snake for going into there.

There are two direct references to Hideo Kojima's previous game, Policenauts, in the game. In Hal's lab where the player first fights Gray Fox, there is a Policenauts poster on the northern wall. When talking to Otacon, he begins to express his love for Japanese animation and robots, showing footage from Policenauts as well.
In the Japanese version of the game, if the player has save data from Policenauts, Psycho Mantis will comment on it.

In the Japanese version of the game, the knocked out guard Spoiler:outside of Meryl's cell will still be wearing underwear. In other versions of the game, the guard is found naked with his rear-end blurred out.

In the second-floor basement in the nuclear warhead storage-building, there is a miniature Moai Head statue in one of the rooms.

If the player shoots the ArmsTech President (Kenneth Baker) during the Revolver Ocelot fight, Ocelot will comment "What are you aiming at?" If Baker gets killed, either by being shot or having the explosive wires tripped, the Game Over screen will have Revolver Ocelot saying "You idiot!"

If the player looks into the room before the DARPA Chief's while climbing through the vents, they can see Meryl doing exercises on her bed. If the player exits back where they came from, re-enters the vents and looks again, Meryl will be doing a different exercise. On the fourth time, Meryl will be back in her original position and exercise, but has removed her pants.

If Mei Ling is called multiple times and the player chooses "Do Not Save" each time, she'll begin to chastise Snake. On the third time, she'll say "C'mon Snake. Don't call me for no reason." On the fourth call, she'll say nothing at all. On the sixth call, she will not give the player a chance to save and will instead stick her tongue out.

Rotating the analog sticks and directional pad while on the title screen/menu will change the colors in the background.

When booting up the game, a short music track will play as the Konami logos appear during the title sequence. The music that plays is actually an extract from a theme called "End of the Dark" from the game Policenauts.

If the player forgets/can't find the SOCOM Pistol before meeting Meryl for the first time, the cutscene of their first encounter will be slightly different; Normally, Snake pulls out his pistol and aims it at Meryl, but being without a pistol makes him put his hand on the tip of Meryl's FAMAS.

The battle with Psycho Mantis includes a pre-fight "demonstration" of Mantis' psychic abilities, in which Mantis will read the player's memory card. If the player has save data from one of several Konami PlayStation games on their memory card, Mantis will reference that game by name. The games that trigger this dialogue are Suikoden, Castlevania: Symphony of the Night, Vandal Hearts and Azure Dreams.

In the Japanese release, both Policenauts and Snatcher are also recognized. Having save data from both of these games will trigger a message from Hideo Kojima, thanking the player for their continued support, but since both of those games were only released in Japan, this message cannot be heard normally in international versions of the game.

There are a number of cut animations first found within the "Pilot Disk" demo, and later found to also be in the final Japanese and American releases. One of them is a jumping roundhouse kick, which would later appear in Metal Gear Solid 2: Sons Of Liberty.

The word Solid in the title has three meanings. The first refers to the main character, Soild Snake. The second comes from the fact that this was the first 3D Metal Gear game. The last meaning is a take on the rival company, Square. As Hideo Kojima said, "A square is a two- dimension thing. And I guess the president of Konami wanted this game to surpass Square. They wanted to make it a cube, you know, like solid 3D."

The Metal Gear Solid: Official Mission Handbook, published in 1998 by Millennium Books, gave an alternate account of Big Boss's early years. This information has since been retconned in more recent games. The guidebook read:

Snake's real name, David, was not a reference to his voice actor, David Hayter, as it was present in the Japanese script long before he was hired to provide Snake's voice. The name is instead a dual reference to 2001: A Space Odyssey and Policenauts.

In Otacon's ending, Snake gives Otacon his real name, and Otacon replies "I'm Hal, Dave." to which Snake jokes they should "take a trip to Jupiter", referencing the Stanley Kubrick 1968 movie and book 2001: A Space Odyssey which starred a main character named "David" Bowman and the famous supercomputer "HAL"-9000.

In Meryl's ending, Snake gives his real name to Meryl, who calls him "Dave". This is actually one of many references to Kojima's previous game Policenauts, in which Meryl Silverburgh was a member of the Beyond Coast Polices Vice unit, and was partnered with a young officer named "Dave" Forrest.

Depending on the selected difficulty level, the number of times that Snake will have to endure torture by electric shock is different, and Revolver Ocelot will have a different comment each time.

• Easy: 3 - "Had enough yet? We're not finished yet!"
• Normal: 4 - "You're still okay? Want me to go on? Can you take one more?"
• Hard: 5 - "Give up. Make it easy on yourself. One more time!"
• Extreme: 6 - "Come on! You don't want to die like this, do you? I can do this all day!"

This was changed in the Twin Snakes version to be 3 rounds, regardless of the difficulty setting.

Inside Hal's lab, there is either a PlayStation with a DualShock controller (in the original PlayStation version of the game) or a Gamecube with a Wavebird controller and statuettes of Mario and Yoshi (in the Twin Snakes remake). Shooting Mario will give the player some health while playing the 1-up sound.

In the 2nd half of the battle, Metal Gear Rex can be defeated with the PSG1.

The production team came up with many designs for the game's areas by building miniatures out of LEGO.

Solid Snake's design in Metal Gear Solid was inspired by the body of Jean Claude Van-Damme and the face of Christopher Walken.

Meryl Silverburgh shares her name with a character from Kojima's earlier game Policenauts, and both Meryls were voiced by the same woman in Japan.

Christopher Randolph, the voice of Otacon, actually first auditioned for the role of Solid Snake. The producers didn't think that Randolph fit, but instead wanted him to try auditioning as Otacon, as Randolph looked almost identical to Otacon's concept art at the time.

When playing on a mono-sound TV, or the audio is set to mono in the settings, calling Colonel Campbell a few times during the Hind battle will cause Snake's team to playfully mock him about not having a stereo TV.

Most of the voice actors were credited under pseudonyms since they did not know if the project was supported by the Screen Actors Guild. The only ones who used their real names were David Hayter (Solid Snake) and Doug Stone (Psycho Mantis).