[[folder:Fallout 2]]
* ''VideoGame/{{Fallout 2}}'' has a healing device called a Super Stimpak, which heals massive damage instantly, but has a timer that inflicts some less-serious damage later on from harsh drug side-effects. It is possible to assassinate the Enclave's president by healing him with about ten of them and letting 2 minutes pass. There is a special text message for doing this to NPCs.
** Along the same line, you can use the drug Jet on the enemies to cripple them before a fight. Jet is a very powerful combat enhancer, but after about 5 minutes of game time, gives them very big penalties. 3 Jet can be used on the oil rig's every soldier, wait about ten minutes, and every soldier will be effectively blind and cannot hit you at point blank range.
** One companion, John Cassidy, has a heart condition. Jet and Psycho works by accelerating the body's systems. Making that ally take either will give him a fatal heart attack.
* The last level in ''Fallout 2'' is supposed to be a running shootout through the BigBad's home base. However, if you have the same kind of PoweredArmor the Bads use, you can drop off your allies at the San Francisco docks and bluff your way through the entire mission, save the battle at the end. Which you can allow temporary allies and base guns to deal with instead of handling it yourself. Sadly this trick doesn't work with random encounters during the rest of the game.

[[folder:Fallout 3]]
* Early on in ''Fallout 3'', you are given the opportunity to do a fairly long sidequest that is essentially an optional, extended tutorial, one stage of which involves investigating an old robotics factory. Another sidequest, normally started much later, involves two "superheroes," the [=AntAgonizer=] and the Mechanist. There is special dialogue for accepting the sub-quest to go to the robotics factory while wearing the Mechanist's costume.
* If you talk to Stockholm (a sniper in Megaton who is normally unreachable) by using the command console to make yourself fly (or manage to climb the way up to him), he will respond that he won't have time watching for raiders and add "[[LampshadeHanging How the hell did you get up here, anyway?]]"
** There are actually ''two'' Stockholms: one for the Megaton city cell itself, and one for the overworld outside Megaton. They are actually two separate entities (albeit with identical appearance and dialogue). Killing one will automatically make the other disappear.
* One of the DLC packs for the game requires you to talk to a few Brotherhood of Steel renegades called the Outcasts. After helping them with a Super Mutant problem, you almost immediately engage in conversation with them. If you happen to be wearing armor that you took from the dead body of an Outcast elsewhere in the game while they talk to you, they start asking what the hell you're doing off-duty.
** Speaking of the Outcasts, there is an unmarked sidequest at their base of operations where the Lone Wanderer can trade certain items for meds, ammo or grenades, if the player has failed to earn their leaders trust by turning in enough technology; Their leader will confiscate any Outcast Power Armor currently carried, stating it was rather convenient that you "found" their armor out in the wastes
* If Lucas Simms dies, the town Megaton's sheriff, his son will have new dialog options with you. Moreover, if you start wearing the Sheriff's outfit, he will reply "You might wear this badge, but you're not my dad." Other [=NPCs=] will react too!
** Lucas Simms will more than likely die, if you side against Mr. Burke in the quest The Power of Atom, during a cutscene of sorts. "Of sorts" indeed, because nothing keeps you from acting during it, and disarm or execute Mr. Burke before he gets the chance to fire his OneHitKill scripted bullet at Simms. The sheriff himself even thanks you for saving his life, praising your quick reflexes.… and your VATS, probably. You have to, however, wait until Burke pulls his gun in order to prevent Lucas's death. The time you have to react is less than a second. Pop a cap on Burke before that, and ''you'' will be punished instead, for interfering with the local law.
* The Power of Atom, in which you either disarm or detonate the nuke in Megaton, is likely one of the first quests you'll get in the Capital Wasteland, and is usually done right away as it results in the only available player housing in the game. Holy Water is a quest that is only available near the end of the game after the main quest is complete and you've begun Broken Steel content, and it starts just outside of Megaton. If you somehow happen to blow up Megaton while Holy Water is active, there is a set reaction from every NPC involved.
** Similarly, if you've already started the Wasteland Survival Guide quest for Megaton resident Moira when you decide to blow it up, upon returning to its ruins you will discover that she was out of town when the bomb went off. The radiation has turned her into a ghoul, but you can still finish the quest for her. However, after a few in-game days, Moira will relocate to Underworld, since ghouls fit in better there… [[OhCrap which can get her killed]].
* If you find James independently of the Galaxy News Radio subquest, either by SequenceBreaking or passing Three Dog's speech challenge, you still have the option to go back and complete said quest, which rewards you with a key to an otherwise inaccessable weapons cache in Hamilton's Hideaway.
* Uncle Roe will tell you that he "hasn't heard from (caravan merchant's name) in a while" if that merchant has died, and you will no longer be able to invest in them. Although if their pack brahmin survives, it will continue to follow the caravan route, and you can [[RobbingTheDead loot the inventory]] if you retrieved the dead merchant's key.

[[folder:Fallout New Vegas]]
* In a mission given to you by The King, you have to investigate a bodyguard named Orris who seems to be doing his job a little too well. He does this by paying thugs to play dead while he fires blanks, giving the illusion of skill. If you decide to go back and attack the thugs on the ground, they get up, yell "Orris 'aint payin' us enough for this!" and attempt to kill you.
** If your character is too dumb to figure out the scam (low intelligence, low medical skill) then The King will be pissed off and make you do it again. Only the next time, Orris won't fall for it and will try to kill you off the bat.
* Faction leaders Mr. House and Caesar tend to congratulate (or scold upon doing something to annoy or anger them) the player on things they have done. Mr. House, for instance, will praise the Courier if they immediately go and retrieve the Platinum Chip instead of talking to him, while Caesar will offer praise them upon killing Mr. House as soon as he enters the Lucky 38.
** After retrieving the Platinum Chip, you are supposed to return to Mr. House. However, if you skip this and instead progress to the point where you enter the bunker in Caesar's fort, he will compliment you on being "ahead of schedule".
** Caesar in particular will give an absolutely epic rant if you've been working against his interests earlier, listing each and every blow you've dealt to his forces and finishing with asking you why you felt it safe to show up in his tent. On the other hand, if you do Caesar's Legion quests before meeting Caesar and dealing blows to the NCR, Caesar will give a more positive rant and note how you and him share a common foe.
* Almost all of Mr. House's dialogue has a corresponding line from Yes Man. If you kill House and install Yes Man into the mainframe before going after the Platinum Chip, it will be Yes Man that greets you inside the Securitron Vault. He will also give the demonstration on what the Mark II upgrade does in House's stead.
** Alternatively, if you pursue the independent ending without upgrading the Securitron army, he will scold the Courier (as best he can anyway, given his programming) for trying to take on Caesar's Legion and the NCR without a standing army.
* Whenever you have a companion, try to give them an order you already gave them a few seconds ago and see the results.
-->'''[[DeadpanSnarker Raul]]''': Right. Got it. I'll just stop using this melee weapon and instead use a melee weapon. [[GladYouThoughtOfIt Good idea, Boss]].
* The NCR's questline has you helping them wipe out the Legion in certain towns. If you've already killed all the Legion soldiers in those towns, the quest-givers will be surprised at how badass you are. Further bonus points [[LaserGuidedKarma if your character is female]], considering [[NoWomansLand the Legion's treatment of women]].
* Bypass Nipton on the way to New Vegas, thereby avoiding meeting Vulpes Inculta. Once you get to Vegas and meet Vulpes Inculta, kill him, then go all the way back to Nipton. A unique Legionaire named Gabban will be in command at Nipton instead of Vulpes that you can talk to instead.
** If you do the same thing, minus killing Vulpes, he will acknowledge that you've met, though he [[CaptainObvious "Wasn't wearing a dog's head at the time."]]
** Likewise, if you kill Vulpes in Nipton, he will be replaced by Alerio on the Strip and at the Fort.
* When you load a game in a casino in VideoGame/FalloutNewVegas, dealers swap decks, croupiers check the wheels against bias and slot machines are resetting themselves for one minute [[SaveScumming as an anti-cheating measure]].
* In ''Old World Blues'', a required quest involves you entering a chamber containing Gabe, the cybernetic dog of Doctor Borous while he was still human. Gabe is hostile and immediately attacks you. You'll almost certainly be forced to kill him, but you're not required to, and even if you dig up the item you need from one of a dozen dirt piles around the chamber, roboscorpions will attack and almost certainly kill Gabe if you didn't. But if you did avoid killing Gabe ''and'' protected him from the roboscorpions (neither of which the game even really hints is possible), Doctor Borous has unique lines thanking you for sparing his dog when you bring Gabe's bowl back to him.
** Speaking of Gabe he has 9 Strength, 8 Agility and Endurance but has [[DogsAreDumb 2 Intelligence]] and, according to the wiki, [[SarcasmMode "a whopping 0 Perception"]], due in part to excessive Psycho in his food. Guess who gave it to him.
* One of the NCR quests involves resolving a conflict between the NCR and the Kings, and one way to resolve it is to kill a King named Pacer. You need to make his death look unconnected to the NCR, so you need to either arrange for him to die by accident or get him to be (or appear to be) killed by the Van Graffs. There's a pretty impressive range of options, such as simply killing him with an energy weapon, spiking his Jet with Psycho to cause him to have a fatal reaction, slipping an energy explosive into his pockets, or even forging a hostile note from the Van Graffs and sticking it on his Jet stash, which will cause him to run off and attack the Van Graffs. The kicker here are two options that aren't readily apparent: if you killed the Van Graffs already, you can tail Pacer to their building and kill him with an energy weapon inside the store. Alternately, you can take another sidequest where your job is to stand guard outside the Van Graff's store in Van Graff armor and using an energy weapon, and then deliberately picking a fight with Pacer when he walks up to the door. There is ''nothing in the game'' that hints that this latter option is possible, but it works perfectly fine for the quest.
** A quest for the Brotherhood of Steel works with this as well - they expect you to kill the Van Graffs, which is kind of really difficult given you can't take your weapons inside and they are all armed to the teeth. It just so happens, however, that one of the [=NPCs=] who wants to enter has bombs strapped to his body - let him in and he'll take care of it for you (though at the cost of some of the potential loot being damaged). An alternative comes right after the portion where you stand guard - since they're paying you to help them, they'll let you take all your weapons inside the shop now.
*** If you DO stop the suicide bomber (which involves killing him) the other guard, Simon, tells you to help clean up the mess so as to not scare off customers. The screen then goes black and the body is removed. However, if you look in a nearby dumpster you can find the body, unless the body has been disintegrated or gooified.
* Weapon Repair kits restore the condition of whatever weapon you have out. If you try to use one with no weapon equipped, you gain a small amount of health, as the game apparently interpreted this as "repairing" your ''bare fists''.
* During a conversation with Johnson Nash, he'll tell you how a cowboy robot had him hire six couriers. This can to an amusing moment where you have to clarify ''which'' cowboy robot he's talking about. Normally the line would be "Cowboy robot? You mean that one over there?" However, if you've spoken to Primm Slim and then went back to ask Nash the exact same question, the line is changed to "Cowboy robot? You mean Primm Slim?"
* During the very start of the game, players will be strongly encouraged to talk with Sunny Smiles to to ask her about any odd jobs that need doing, which segues into a tutorial about hacking/lockpicking an old safe. If the player happens to wander into the schoolhouse and crack the safe beforehand, you have an option to tell Sunny about it, in which case she'll just let you keep the bobby pins, since they'd clearly be of better use in your hands than hers.

[[folder:Fallout 4]]
* The dev team included a database of 1000 names that Codsworth can call you by as opposed to simply "Sir" or "Mum". These names range from common names such as Abigail, Dan, and Joe; to pop culture references such as [[ComicBook/{{Superman}} KalEl]], [[Film/MadMaxFuryRoad Furiosa]], [[Book/TheHungerGames Katniss]], Manga/{{Akira}}, [[Manga/RurouniKenshin Battosai]], and [[Anime/{{Trigun}} Vash]]; all the way to more profane names like Fuck, Fucker, and [[RuleOfThree Fuckface]].
* At any time once you reach Diamond City, you can go to a [[MagicPlasticSurgery plastic surgeon]] to essentially re-enter the character creator and redo the Sole Survivor's face completely. Later on in the story, however, where you [[spoiler:enter Kellogg's memory and see "yourself" in his mind back when he abducted Shaun, your face will still look the same as it did the first time you created your character.]] A small touch, but nice attention to detail.
* Sturges, a pretty major character in the Minutemen questline as their resident tech expert, is a Synth. This is never remarked on in game, and he's not killable, so normally you wouldn't find out. Players only discovered he was one by turning off his essential status using the console.
** Sturges comments if you first meet him wearing power armor. You're meant to get your first set after meeting him.
* Reading Raider terminals you'll quickly find that the Raiders take note of the other bands the Sole Survivor wipes out, and have a pretty intricate relationship to one another.
* A minor one: you can place down televisions in Settlements, and can even get them to turn on when you supply them with power. As you can probably guess, all they do is play the standard "Please Stand By" screen.
* There's a rare bug where kill animations that for whatever reason fail to kill you can sometimes destroy your limbs as if you'd died. But you'll walk around with the exploded body parts floating around with you. What makes it this trope is that they actually programmed in effects in game for the player missing various limbs. Losing your head drops your perception down to zero, losing a leg will drop your agility to zero, and so on and so forth. Similarly, stimpacks no longer heal the missing limb.
* If you rest the crosshair on Cait's buttocks a few times, she will remark about you leering.
* When you switch out companions, they will interact with each other. Some of them like each other (like Piper and Cait) and will banter in a friendly manner, and some of them strongly dislike each other (like Danse and Strong), and they will coldly snipe at each other.
** On the topic of companions, [=NPC=]s have an impressive amount of unique dialogue depending on where you are and who your current companion is. Diamond City guards will [[https://www.youtube.com/watch?v=MNkvc5ybdb0 react]] differently based on your companion, and your companions will engage in conversations with [=NPC=]s such as [[https://www.youtube.com/watch?v=9dx37_RsL3I Whitechapel Charlie]] and [[https://www.youtube.com/watch?v=jMH9-qGc680 Takahashi]].
* If Dogmeat is with you, you will occasionally hear the raiders warning each other that you have a dog when combat commences.
* Diamond City will redecorate on in-game Halloween and Christmas Day! Halloween DC will be decorated with various pumpkin posters and children's costume advertisements, while Christmas DC will be covered with Christmas trees and colored lights. Even the guards will throw a comment, if a rather bitter one.
* Averted with [[spoiler:Kellogg's memory sequence]]. Skip through the [[spoiler:memories]] without watching them and you'll still have the dialogue option to [[spoiler:bring up [[AlasPoorVillain how shitty a life Kellogg had]]]] with Dr. Amari.
* If you reach the point where you have to track [[spoiler:Kellogg, your son's kidnapper]] without having met Dogmeat, Nick Valentine will call "someone", calling him a "specialist", and whistle in a frequency this "someone" will be. And this someone is, [[spoiler:you guessed it, Dogmeat!]]
* Completing the post-ending quest ''"In Sheep's Clothing"'', [[spoiler: in which Mayor [=McDonough=] is exposed as a synth agent for the Institute]] before getting Hancock's affinity with you to the halfway point will alter his heart-to-heart with you, expressing his frustration with whether his hatred for the central character of said quest was rightful, misguided, or even had any point at all.
* Tapping a button in the middle of a conversation to skip the line will frequently prompt your character to go "Uh huh", "Yeah?", "Okay", and stuff like that. Pressing the button that prompts that's usually mapped to a certain kind of response will have your character interject more rudely, neutrally, or positively. For example, the negative response (B for Xbox One, Circle for [=PS4=], Right on PC) will have you press your interviewee with muttered insults.
** The rude interjections are also specific to whatever kind of character you're talking to. For example, rudely pressing while a Robot character is talking will have you mutter "Crazy ass robot..." or something like that, while talking to a senior will have you say "Stupid old man..."
** Additionally, your interjections change depending on whether or not you're on chems or alcohol. Interjecting while high will frequently result in you say something in a very breezy "Yeeeeah..." or something, while doing so while drunk will appropriately have you interrupt with a slurred "Yyyesssss..." or ask to keep it down (implying you're hungover).
** If you have low Charisma you will noticeably be hesitant or stutter when you speak.
* In the prologue, if you're stubborn and refuse to fill out the Vault-Tec forms, [[ButThouMust your spouse will do it for you]]. Also, if you spend too long to get into the Vault when the bombs start falling (or try to leave the neighborhood), it results in a NonStandardGameOver as the nuke falls and obliterates the area.
* On the Brotherhood of Steel's questline, you find that they are trying to rebuild Liberty Prime - the {{giant robot}} from the climax of ''Fallout 3'' - and because she was instrumental in repairing Prime last time, they send you to recruit Doctor Li away from the Institute. If you've had yourself banished from the Institute, Elder Maxxon wil chew you out for it and switch to the back-up plan of recruiting Professor Scara from Diamond City's Science! Center.
** It's clearly implied that Scara and Doctor Duff of the Science! Center are in a romantic relationship, so if Scara leaves to work with the Brotherhood, Duff will be noticeably saddened by her absence.
* It is possible to become effectively immune to damage by wearing six pieces of armor with the Sentinel legendary effect and wielding a weapon of the same effect. [[GameBreaker This grants you 100% damage reduction when standing still]], and has an interesting effect on the AI of the game. Stupid and unintelligent enemies, like animals, feral ghouls, and super mutants, will attack you no matter whether you're invincible or not. However, human enemies will recognize that you're bulletproof when you're not moving, and run away screaming in fear.
* The Railroad quest "Memory Interrupted" eventually leads to you and fellow heavy Glory getting assigned to clear the same area independently of each other, due to the organization's compartmentalized structure. If you bump into her on the way out of headquarters, she'll remark that she's on her way out on a mission instead of her normal idle dialogue.
* The DLC ''Far Harbor'' has an impressive number of these. For example, it is possible to bypass a speech check in one quest if you happen to have the [[ImAHumanitarian Cannibal]] perk.
* Hancock will typically join the player as a follower after the player has redeemed themself by killing their heist partner. There is an alternative scenario if the player constantly acts as a thorn in Hancock's side. Should the player take part in the heist, kill Hancock's Seargent, spare his/her heist partner, admit to Hancock that they spared her WHILST giving the reason that "It didnt feel right"; then Hancocks reaction and subsequent decision to follow you are different. Hancock will be outraged but quickly calm down and recognise the irony in the situation. Hancock spent his life disregarding authority, and by constantly disobeying Hancock he realises he has become 'the authority' and thus opts to join you to rekindle his role as a rebel.
* Interestingly, it's possible to avoid recruiting or talking to Codsworth until ''after'' you've completed the main questline. Doing so causes the tone of Codsworth's recruitment dialogue to change significantly, as the Sole Survivor admits that [[spoiler:Shaun is dead]]. Codsworth then decides to join out of pity more than anything else.

* Surprisingly averted in ''VideoGame/FalloutShelter'': you can very well create a vault with the ID of 0, and the game will not complain whatsoever.
** Alternatively, they ''did'' predict someone using 0, because [[VideoGame/FalloutTacticsBrotherhoodOfSteel there actually is a canonical Vault 0.]]
* If you assign a pregnant vault dweller to train her Charisma at a bar, [[OneDrinkWillKillTheBaby she actually doesn't drink beer like the others, she drinks Nuka Cola.]]