The Dev Team Thinks Of Everything: The Elder Scrolls V: Skyrim
NPCs will interact with items you drop on the ground, and depending on the value of the item, how wealthy the NPC is, and if they're friends with you or not, they may just take the item or come to tell you that they saw you drop it and will offer it back. If you drop a load of weapons near guards, they'll tell you to knock it off because leaving them lying in the streets could get someone hurt, and if you're a jerk about the warning, they issue you a small bounty as a fine for disturbing the peace.
It also works as a distraction with enemies: Drop a valuable gem next to a group of bandits and they'll start to argue who saw it first, giving you an opportunity to sneak past them. Or, if you prefer to, you can wait until the fight turns violent, and then clear out the survivors.
Using a Thu'um in a city will cause a nearby guard to insist that you stop because you're making people nervous. Then if you challenge them to prove that you're actually breaking the law, they are forced to sheepishly admit that there isn't one, but stop doing it anyway!
If, however, you're under attack and use a Shout, then the guards not only won't bother you about it, they'll actually help you fight.
Knocking stuff over near people will cause them to chide you for carelessness. Running into people will cause them to complain. Running into a companion who is blocking a door or ledge will cause them to mutter an apology and back out of your way.
If you knock a sign around, it can swing back and hit you in the face for minor damage.
The game tracks who saw you commit a crime. If you kill the last witness, any bounty incurred will be removed. A hilarious example is, if you kill a chicken near a house in the woods, if it's alone, a bounty will show up and then instantly removed.
You can "pickpocket" the Briar Hearts out of Forsworn Briarhearts; doing this kills them instantly. Unfortunately, the dev team didn't think it through fully, and you might get accosted by hired thugs sent to "teach you a lesson", that were hired by the Briarheart.
There are variations in dialogue between characters depending on if you spoke to them before other characters in the conversation got to them. For example, early in the game, if you get to Hadvar's uncle first and talk to him before Hadvar does the conversation goes a little differently; (i.e., "Yes, Player Character told me about the dragon too and I hardly believed it."). NPCs will also have different dialogue if you've already looted the objects required to complete their quests.
Similarly, if you've killed Grelod the orphanage ownerbefore starting the Dark Brotherhood initiation quest, Aventus Aretino will be amazed at the guild's efficiency when you go to talk with him about his Black Sacrament.
You can snatch arrows out of the air in mid-flight, and knock arrows out of the air with your own. Nearly impossible to do either of these without the Slow Time shout, but pull it off, and you'll feel like an absolute badass. You can even grab an arrow out of the air, equip it, and shoot it back.
During the mission "Diplomatic Immunity", you can disguise yourself as a Thalmor officer. The effectiveness of your disguise, however, is actually based on your race: Beast-races will be spotted instantly due to their vastly different anatomy, humans will be alright at a distance but will get caught if too close and non-Altmer elves can get a little closer. Altmer, of course, have it best, as not only can you walk freely all the way to the Solar, you can actually talk with the guards and order them away from their posts. At no point is this tactic ever suggested to you, leaving it purely up to your own initiative. The only catch is you need the Hooded variant to conceal your face from a distance if you're not Altmer. Interestingly the guards have dialog for checking your face even if you're wearing a hooded thalmor robe.
Sit down in a tavern and the waitress will automatically approach you and ask what you want to drink.
When exploring a crypt with a warrior whose family was buried there, he protests if you start looting, but lets it slide since you're helping him clear out a necromancer doing far worse things to his ancestors' remains.
A merchant in Riften is in debt to Sapphire, a member of the Thieves' Guild. The player can persuade her to let him go, if they've joined the guild they can demand a cut of the profit, and if they've become Guildmaster, they can pull rank and just tell her to back off. The latter option particularly has her grow nervous when you bring up the subject because you've found out she's doing business on her own without giving the Guild a cut.
Skeletons only get jostled by Fus Ro Dah because most of the force doesn't actually hit them but simply passes through the empty spaces between their bones. Draugr, which have preserved skin, however, can be knocked back with the full force of the shout.
In the Civil War questline, once you prove yourself to either Tulius or Ulfric and take the oath to join them, you cannot just switch sides by changing your mind and take the oath to join the other side instead. The quest immediately after is to find the Jagged Crown, and once you have it you have to take it back to your leader. However, as an optional objective never hinted at, you can deliver it to the other leader in exchange for letting you switch sides. And they comment on you trying to play both sides, and the next quest, defending or besieging Whiterun as the first major battle of the war, shifts to a test of loyalty to make sure you're sincere.
All the children running around the cities? You can play tag and hide and seek with them. In some cities, you can win by hiding in a place children aren't allowed to enter (e.g. Castle Dour courtyard).
You have to loot body parts from certain enemies, like the Glenmoril Witches. If the body part is visible, the ragdoll changes.
You can shield bash by attacking while blocking. If you're blocking with a torch, this will set the enemy on fire.
If you're in the middle of being arrested when a dragon attacks, the guard will say he has more important things to do and run off to fight.
The quest to formally join the Dark Brotherhood has you ordered to kill one of three bound figures, one of which has a contract on them, but you're not told which one. You're free to kill any of them you choose, or to kill two of them, or to kill all three of them. The questgiver has unique dialogue to respond to each of these different courses of action. You're also given the option to kill her instead, and can then free the captives.
Casting Calm or Courage on an NPC is a good way to grind your Illusion skill. However, try casting Calm on Colette at the College of Winterhold; her response will make you feel dirty.
Colette: Stop that! I will not be violated by some...half-Septim magician!
When going to the College of Winterhold, Faralda won't let you pass without either demonstrating your aptitude at magic or passing a persuade check. However, if you're far enough into the main quest, you can simply tell her you're the Dragonborn and fire off a shout, which impresses her enough to let you through. This is because the Thu'um is an ancient, powerful and nearly-forgotten branch of magic, one which few people still study, let alone master; making you somewhat uniquely qualified in the field.
Look in your inventory after you get hit by a barrage of arrows. They apparently stick in you, so you get to keep them. This also happens to NPCs with arrows you fire at them.
A quest in Morthal has you hunting down a vampire who's been living in a the town for a while, if you try to confront her and you're a vampire yourself, she says she knows you're one in the same and snidely tells you keep it a secret "that there are wolves living amongst the sheep."
A similar interaction occurs with Arnbjorn when you join the Dark Brotherhood. He's a werewolf and he'll know you're one too if you've done enough of the Companions quests. This special dialogue does not appear otherwise.
If you proceed far enough in the Dawnguard questline to meet Durnheviir without completing "Dragon Rising" (that is, killing your first dragon and learning you're a Dragonborn), he comments that it was his first instinct to refer to you as a "dovah" but he's not sure why, and perhaps you'll find out for yourself in time. Similarly, if you've completed the main quest, Durnheviir will acknowledge your triumph over Alduin.
Upon joining the Legion, General Tulius will recite an oath that the player makes his character repeat to confirm joining up. The pledge swears allegiance to the Emperor, Titus Mede II, however if you completed the whole of the Dark Brotherhood Quest Line before starting the Civil War quests, the general just says "The Emperor" rather than his name
For the thieving killing types, joining the Thieves Guild and Dark Brotherhood at the same time would be an ideal goal. Join the Thieves Guild beforehand and get acquainted with Delvin Mallory. After you've joined the Dark Brotherhood and received an amulet which Astrid tells you to show off to Delvin, he will remark how you're making friends all over the place and comments on your work as an assassin.
When you first enter Markarth, a small scene will trigger where a visitor to the city will be murdered by an agent of the Forsworn. If you're fast enough, you can kill the agent before he gets to her. Talk to her, and she'll thank you for saving her life and reward you with a piece of jewelry. Later, during the quest "The Forsworn Conspiracy" (which is started after said scene), you're able to ask her directly for information, rather than having to search her room.
During "A Blade In the Dark," Delphine will talk about Kynesgrove and the dragon burial mound there. On the off chance that you actually made your way all the way out to Kynesgrove and went all the way up to the top of the hill to where the burial mound is located prior to the quest, then when Delphine brings up Kynesgrove, you get a unique dialog option to mention that you've seen the mound - but not if you just visited Kynesgrove. You have to actually walk all the way up the hill to the mound itself to trigger this.
Towards the end of the game you have to declare your worth to a guardian before he'll let you pass. Depending on which faction quests you've completed, you're given different choices, and he comments on each one. And if you haven't done any, you can simply claim that you're Dragonborn.
If you completed the Civil War in favor of the Imperials before completing the Main Quest, Ulfric and Galmar will later appear in Sovngarde. Galmar still hates you for what you've done. Conversely, Legate Rikke will appear if you complete the Civil War in favor of the Stormcloaks. Kodlak Whitemane can make an appearance if you've completed the Companions quest line.
In Dawnguard you're accompanied in one dungeon by the ghost of a former adventurer. When you find her corpse, she suggests taking her journal for help. If you loot the corpse's armor as well, she complains about it and asks you to let her have her dignity.
Necromage, a Restoration skill perk, increases the effects of all spells on undead. If the player is a vampire, their self-targetting spells will also be more effective.
Almost every single door that is opened via a mechanism (lever, pull chain, secret handle, etc) has a mechanism on each side of the door, so it can be opened from either side. This is even if you couldn't logically get to the other side of the door from any in-game method. This is probably a failsafe in case of glitching or intentional clipping through walls getting the player trapped in an unwinnable situation.
More likely it's a nod to realism; the place's builders would logically have put mechanisms on both sides. As for a given room's present state, there's no way they could've predicted that.
As the switches also close the doors, this lets a truly obsessive-compulsive player close the door behind them, then open it again when they're ready to leave the room.
If you have completed Discerning The Transmundane, read the Oghma Infinium, and agreed to be his champion (or not), when you enter Apocrypha in Dragonborn, Hermaeus Mora will greet you as such, referencing each event, and future conversations will also reflect your champion status, all the way through the main storyline. Although, if you have encountered a black book prior to the point in the main storyline where you're "supposed to", there is no special dialogue for that.
Likewise, completing Dragonborn and then doing Discerning the Transmundane will give Hermaeus Mora dialogue saying he's tracked your progress since Miraak's defeat.
If you've acquired the Oghma Infinium, and talk to Neloth in Dragonborn about Hermaeus Mora, you can genuinely impress him by saying that you've read the book.
Falmer can be extremely difficult to fight, as they gang up on you, are very durable and come with powerful weapons. However, they become laughably easy if you are a character that uses stealth. Why? Because Falmer are blind. They don't see targets, they hear them. This means that, so long as you don't make any noise, bump into one and kill them in one hit, they become some of the easiest enemies in the game.
There are several quests where you can complete the objective before you are given the quest, such as Farengar Secret-Fire asking you to retrieve the dragonstone from Bleak Falls Barrow. If you already have it, he will complement you on being smarter than most of the grunts that visit Dragonsreach.
Various bosses will comment on the player's behavior during the fight. For example, turning invisible when fighting Miraak causes him to taunt "hiding is beneath you, Dragonborn", and using Dragon Aspect has him note that you've already mastered his own Shout to use against him. Using the Dismay shout on Alduin (which makes targets flee as though Fear was used) has him laugh "do you think I will run, Dovahkiin?", which heads right for Hypocritical Humor since that's exactly what Alduin does when you beat him the first time.
Not only can you kill children, but certain children are flagged to send you letters thanking you for killing their other child rivals, just like adults!
In Dawnguard, if one is already a vampire when recruiting Serana, her dialogue changes to reflect this, wondering if her father sent you and then being confused when you admit you have no idea who Lord Harkon is.
Due to the well-known glitch that a killing blow from a Giant's club can send people rocketing into the stratosphere, when Bethesda and Valve teamed up to release the Space-Core mod, they made it so that if the Space-Core is struck, it will cheer as it sails off and is permanently lost from the game.
Once Harkon is killed, you can travel back to the Soul Cairn and tell Valerica. She will then return home to take up residence in her study.
If you have Erandur (a noted priest of Mara) as your follower, and head to the Temple of Mara in Riften, he'll remark that he's always wanted to make a pilgrimage there, and that you've made him happy by bringing him.
If you enter a business and start snooping around the place, the owner will drop what they're doing and follow you to make sure you don't steal anything. Particularly frustrating for thieves.
During the prologue, Ralof makes a passing comment as he enters Helgen to be executed. "I wonder if Vilod is still making that mead with juniper berries mixed in." If the Dragonborn returns to Helgen later in the game and jumps from the tower onto the second floor of the inn, like they did in the prologue, they'll find a few bottles labeled "Mead with Juniper Berry".
You can mine for ore by attacking the ore vein with a pickaxe.
If a dragon attacks while you're talking to Meridia, Meridia will actually start fighting back.
If you go all the way back to Ustengrav after completing "The Horn of Jurgen Windcaller", you can return the horn to its resting place, which will grant you a free Dragon Soul.
When you first meet J'zargo, he'll ask you if you've yet mastered the Expert-level Destruction spells. Normally, you can only say no, causing him to gloat that he'll be the first. However, if you take the Expert Perk in the Destruction Tree, you can tell him that yes you have. J'zargo is legitimately impressed, and a little surprised.
Similarly, if you hurry into Farengar's quarters after sending news of the dragon attack to Jarl Balgruuf, you can ask about his dragon project before the Jarl comes in. If you have raised your enchanting, destruction, or alchemy skill high enough, you can show off your knowledge of said skill by using jargon that only someone proficient in that skill would know. Like J'zargo, Farengar is impressed by your knowledge.
If you get married, the game keeps track of people you've helped, become guildmates with, and/or become close allies with and people like that will be at your wedding, even if they have to travel from another part of the game world to do it.
If you order your follower to interact with an All-Maker Stone that is still corrupted by Miraak's influence, they'll enter a trance and begin working on the shrine/temple like everyone else, complete with reciting Miraak's Mantra.
Approach Captain Aldis while wearing Stormcloak armor and he'll ask you what the hell you're doing. You can apologize and end the conversation peacefully, or proclaim your loyalty to Ulfric Stormcloak, causing him to try and kill you.
After activating Dragonborn's main quest up to Miraak's introduction, he'll randomly show up and Kill Steal a dragon soul from you. When you finally end him, he'll give you back ten dragon souls plus however many he stole in the interim.
If you side with the eponymous group in Dawnguard, you will be sent to recruit a Nord hunter, who will be waiting outside a random bear cave. If you had cleared out that cave previously, the hunter will complain about you killing the beast he spent two weeks tracking.
If you ask Viarmo about his opinions on the Civil War, his dialogue changes slightly depending on the war's current status. For example, if the Stormcloaks have won, he'll say, "Even with Ulfric here in Solitude, there are no winners to be had. No real conclusion."
In the Dark Brotherhood questline, you must give Astrid a sealed letter that contains the plans to kill the Emperor. If the player decides to read the note before handing it over, she'll later mention that the seal was broken, and admonish you for being so sloppy.
When Brynjolf provides a distraction to get the merchants' attention at the beginning of the Thieves Guild Quest Line, the Last Dragonborn's follower will also be distracted.
A very obscure one. If your follower somehow manages to get in trouble with the guards, such as accidentally hitting one, they'll complain about the mess you got them into. You can assure them that you'll handle the guards.
During Sheogorath's Daedric quest, you can't directly open the character menu. However, if you manage to access your inventory by pickpocketing Pelagius or searching the dead goat, you'll find a unique item description for the Wabbajack: "It's all you've got..."