The Gith artifact that prevents the playable cast from turning fully into Mindflayers looks a bit like a spikey d20. It makes for a cheeky almost meta way to justify the heroes being safe if you look at it that way, since the artifact could be argued to have "rolled" in your favor.
The party's reactions to Dribbles the clown in Act 3's Circus of Last Days:
Most of the characters express some level of apprehension to Dribbles the clown—except for Wyll, who admits that he actually loves clowns. While the majority of your companions would have little to no experience with clowns, Wyll was raised as a noble in one of the largest cities in Faerún and would have had plenty of occasions to go see them, naturally making him less afraid of them.
Also, Wyll is the only Charisma focused Origin Character by default, meaning he'd have an easier time appreciating a clown, even being one of the only ones who gains Approval if you send him up to the stage, while the rest of the party have at best only above average Charisma stats by comparison.
To further prove this point, a Baldurian Tav/Dark Urge can express a similar initial reaction to Dribbles precisely because they've seen this routine a lot as someone who's lived in Baldur's Gate for most of their life.
Similarly, Jaheira also laughs at Dribbles' pun, and actually gets defensive about the fact she enjoys their stuff. Not only is Jaheira much older than most of the party (at least outside of the other elves), she's a Baldur's Gate native who has also raised many kids in the city, so has likely been to see Dribbles in the past on multiple occasions. Being exposed to children's entertainment consistently is liable to either make you love or hate it, and evidently with Jaheira its gone the former.
Ascended Astarion ending things if you choose to become a Mindflayer isn't just him being shallow. He only wants a "relationship" he can have complete control over, with a spawn he made and be able to compel once the soon-to-be-dead tadpole's out of their system, and he'll reject any attempts at compromise despite respecting you more for refusing him. As a Mindflayer you'd no longer be his spawn, as Karlach's case proves it overrides her engine when initially she had hoped it would have also killed the resulting Mindflayer. A Mindflayer may not be his equal power-wise, but it'd be a lot closer to it than you could have been and it is more cumbersome/dangerous to keep fed than a typical vampire. Being unattractive is just the easier and safer excuse to use than risk making you aware of the threat you might be to him, though his other path shows it is also a genuine factor.
Non-ascended Astarion will also break up with mindflayer Tav, though he will imply that he may be interested in starting up the relationship again once he grows more used to their new form. This also isn't just him being shallow/put off by your looks; he mentions directly that he isn't really sure if you're still... well, you anymore. After a lifetime of forcing himself to be intimate with strangers, it seems he wants to just be friends for long enough to make sure his lover is still in there somewhere before resuming their relationship. It however seems he feels pretty doubtful that's the case, his utter devastation and the general morose tone he takes implies he's already mourning you. And given how Karlach is in her mindflayer ending, he probably made the right call.
Why was the Dark Urge chosen as Bhaal's favorite child? Aside from being made from Bhaal's blood rather than via copulation, they're also the child with the highest potential to pull off murder on a grand scale, as shown in the Sin of the Father ending — a feat that Sarevok and Orin could not accomplish.
During the "Necromancy of Thay" quest, the correct option to address the Mirror when they ask about Szass Tam is that he's a monster and scum. This despite the fact that the owner is a wizard of Thay themselves. This is easy to figure out, though, if you note the wizard is living in the middle of nowhere with their base hidden underneath an apothecary shop. If they were allied with Szass Tam, the Wizard King of Thay, they would be in much better circumstances.
An in-universe one happens with Shadowheart's flashback to her childhood and being "rescued" by Sharites. You can point out that a child running through the woods with a Selunite pendant is very likely to have been stolen rather than rescued. It's also possible that the wolf was guarding her rather than attacking her.
Compare the three magical projections in the game. Lorroakan isn't a good wizard, and his projection can only accept specific words as prompts (such as SHOP or TAX). Rolan is stronger than Lorroakan, and his projection can understand basic phrases (such as "I want to shop" or "show me your wares."). Gale is a former Archmage, and his projection can understand off-the-cuff sentences, such as complaints about his convoluted security protocol.
Mystra sending Gale to Suicide Attack the Absolute makes a lot more sense than it initially appears. Not only is it a Fantastic Nuke to get rid of an existential threat to the world but it is very specifically a test of Gale. His crime was hubris and attempting to gain godhood out of a selfish pursuit of power (justifying it as impressing Mystra). Showing he's willing to sacrifice himself is a way to show he's genuinely learned his lesson. The request also has less of a sting since Mystra and Gale both know an afterlife exists as well. This makes an extra level of sense when you consider that the punishment for hubris being death is exactly what happened to Mystra's previous incarnation, whose hubris led to her believing she was too important to have to listen to Lord Ao, which later got her killed by Helm when she attacked him in frustration. Not only that, but if he does it that effectively deals with two crises on the surface, getting rid of both the Absolute and the Netherite bomb inside him with minimal collateral considering the sheer scale of both threats.
Orin can only kidnap and replace Gale, Halsin, Lae’zel, Minthara, or Yenna. Why them? Everyone else is too risky.
Astarion doesn't have a reflection, so his double could be spotted with a hand mirror. He's also the target of a powerful vampire lord, which is likely more than Orin wants to deal with.
Karlach's Infernal Engine lights her on fire every time she gets worked up; while reproducing this is doable, it's more trouble than it's worth.
Wyll's patron Mizora has a live camera feed, and already hates the Absolute. There's no guarantee she won't tattle to the party. note While it's entirely possible for Mizora to refrain out of spite, it's enough of a possibility for Orin to not take the risk
Jaehira has family in the city, and they'd be harder to fool than the party.
Jaheira also knows Minsc (and to a lesser extent Volo) too well to be fooled by a doppelganger.
Depending on player choice, Shadowheart is either welcome to visit the people who raised her, or they have an active hit on her.
Dame Aylin and Isobel know each other too well. There's probably little to no intel on how they behaved in everyday life with each other, and it might not even be possible to reproduce Aylin's powers.
The Artifact clings to the PC. If Tav or Durge were taken, the Artifact going missing, being replaced with a fake, or suddenly clinging to someone else would rouse suspicion.
On the flipside, the characters that Orin can kidnap and replace are of lower risk. Why is that?
Gale is rather eccentric, wears his heart on his sleeve, and by the time Act II rolls around, everyone at camp is already completely aware of all of his baggage. He is such an open book that impersonating him requires little to study.
Halsin has spent less time travelling with the party and does not have an established dynamic with anyone else. In addition, Halsin was kidnapped by the Absolute and spent some time in captivity under Minthara, who is a chosen enforcer for Orin, meaning she probably has some intel regarding Halsin's personality and mannerisms.
Minthara for the same reason as Halsin, alongside the fact that her and Orin were (likely) sexually involved at one point, meaning that Orin quite literally knows her inside and out.
Lae'zel is quite prickly and keeps herself closed off to the rest of the party, making any odd behavior from her end easier for everyone else to brush off as what they assume is Gith behavior.
Yenna, being a little girl, poses the least chance of fighting back out of all the targets. However, she will only be kidnapped if none of the other options are available. Given that she's among the last to join the camp, Orin probably figures that Tav/Durge is less attached to her so she's not as likely to tempt them into cooperating. (The fact that most decent people won't leave an innocent child to die if they can help it probably doesn't cross her mind.)
If Astarion and Jaehira walk around her house, he mentions that he was warned away from this neighborhood for some reason, and Jaehira says it's probably because she killed the last spawn she caught lurking around her house. Jaehira also lives down the road from the House of Grief, so it's also possible that Cazador didn't want to piss off the Sharrans.
It might seem odd that Origin Shadowheart can choose to spare the Nightsong without Tav to talk her down, or any objection from anyone else in the party. However, if Tav that's been 100% supportive of her Sharran faith and Judiciar dreams leaves the choice entirely up to her, zero objection whatsoever, Shadowheart will still abort mission and toss away the spear. Shadowheart has so many doubts in that moment (Aylin outright says later that Shadowheart finally listened to her heart) that will only go through with killing the Nightsong when actively encouraged to do so, so if anything it makes more sense for her to bail without Tav there to goad her on.
The Beno Boys were searching for the Nightsong in the wrong place, but their intel wasn't entirely wrong. Indeed, she was found in an abnormally dark place separate from the surface (Shadowfell, Underdark), beneath a forgotten temple to a night goddess (the Gauntlet of Shar, the desecrated temple of Selune), which was located near an abandoned town of Selunites (Reithwin, Moonhaven).
Gale is one of the easiest companions to gain rapport with in Act 1, as he approves of basically every player choice that isn't being a total dick. This makes more sense after his later reveal that he'd spent the past year mostly alone, locked in his tower while he dealt with his condition. The player character is the closest he's come to making a friend after a year of isolation, so it makes sense that he would go overboard responding positively to even the slightest displays of kindness.
Gortash is the only of the Chosen who can outright ally with you, despite being evil. This lines up with the characterization of the Dead Three: Bhaal and Myrkul have no redeeming traits at all and are evil without any degree of standards or principals to their methods, while Bane is willing to be pragmatic and focus on the goal over being unnecessarily evil at times. Gortash, being the brains of the Absolute, knows things have to be changed on the fly if necessary, so being pragmatic enough to ally with the player not only is smart, but fits within what Bane would support as a way of maintaining control over the Netherbrain.
Cazador's Manor is located in the Upper City, but has a secondary entrance to the Lower City. Given how much the Flaming Fist love their checkpoints, wouldn't this be illegal? Yes, and that's the point; Astarion and the other spawn hunt in the Lower and Outer Cities, and they used this door to smuggle 'riffraff' into the Upper City. The Charmed guards aren't just extra muscle; they were probably Charmed to ignore the illegal passageway, as well as stop other criminals from looking.
If playing a good aligned Dark Urge and at Gortash’s Coronation, saying you’re there to end the plot surprises Gortash, with him stating: "One’s true nature always rises to the top", suggesting he knows the Dark Urge better than they do. Considering (if you play an Oathbreaker Paladin) it’s mentioned the Dark Urge wasn’t always a murderous lunatic, and with the game drawing attention to the idea that one's lineage or blood does not mean their true nature, it provides a good reason why even an Oathbreaker Dark Urge can become a force of good; deep down there is the possibility the Dark Urge was once a good person before Bhaal's influence became overpowering.
The Emperor taking the form of the Dream Guardian.
By default, the Dream Guardian is a female Lolth-sworn Drow, who are generally considered to be Always Chaotic Evil in-setting. He's trying to ease the player into accepting him as someone who appears to be evil at first glance.
The Dream Guardian also can by any race but Dragonborn. Perhaps the Emperor's relationship with Ansur soured too much.
Karlach mentions that her guardian is a 'golden Paladin'. Given that she spent a decade being betrayed and backstabbed in Avernus, she might be more inclined to trust someone who literally cannot betray her.
Gale is the only one who mentions the gender of his Guardian, referring to them as a "her". Given his backstory, it seems rather likely the Emperor may have taken some cues from Mystra, as powerful, unknowable women are clearly a fancy of Gale's.
Astarion mentions the Guardian being beautiful, but it's largely glanced over in his post-dream dialogue. He gushes far more about the Guardian promising him power, which sounds very different from how the Emperor speaks to the player in their dream. The Emperor clearly put a heck of a lot more emphasis on the power they can get from the tadpoles, something he knows Astarion is desperate for to protect himself.
Raphael's stats are a bit inconsistent, but he generally has only above average Dexterity, Intelligence and Wisdom, but high Strength, Constitution, and Charisma. The former stats make it appear that he isn't as clever, smart, or crafty as he projects, while the former lets him project a sense of strength and charisma that he doesn't actually have. One of the easiest ways to get under his skin is by showing yourself to be more clever and crafty than he is, which is put on display by sneaking into the House of Hope without his permission, and managing to get Hope, as you've managed to show yourself as more clever and intelligent than he thought.
A journal in the Shadow-Cursed lands implies that aspiring Dark Justiciars made use of the Nightsong's immortality to repeatedly sacrifice her as the final step to becoming a Justiciar, yet Shadowheart is able to kill her for good. Should Shadowheart instead choose to spare her, the Nightsong is surprisingly easy to free from her prison. Why? Ketheric spurned Shar in the century in between the previous visitor and Shadowheart, and Shar is nothing if not spiteful (just ask Shadowheart when everyone leaves the Shadowfell after sparing the Nightsong). Should the Nightsong be killed, Shar is undoubtedly powerful enough to infuse the Spear of Night with the divine power needed to kill the daughter of her rival goddess; part of the item description all but states this, saying Shar's Blessing allows the wielder to kill the Nightsong "If Shar allows it". If the Nightsong is spared, since the Shadowfell is Shar's domain, she's undoubtedly able to alter the magic surrounding the cage enough for the Nightsong to be released, likely thinking along the lines of "I'll deal with Shadowheart in a second, I still need to make sure the other traitor dies."
Astarion's charisma stats make more sense with his backstory when you take a look at his skills; while his charisma is a perfectly average 10, he has decent buffs to his deception and persuasion skills due to his class and background. Astarion may not be brimming with natural charisma like Wyll, but he is a extremely practiced charmer.
One of the special benefits of the player's illithid tadpole would be hinted with the player's Drow and Duergar (or picked Astarion origin) lacking sunlight sensitivity while non-playable characters had sunlight sensitivity. This also plays into the game's epilogue party being set at night as the lack of tadpoles meant that they would feel comfortable (or in Astarion's case, survivable) without sunlight.
Everyone's reaction to the Dark Urge dying after spurning Bhaal, only to be resurrected by Withers, seems much more subdued than you'd expect from people who just got a front row seat to their leader, friend, and possibly lover for one of them dying. However, it makes more sense once you've completed Arabella's questline. Withers soothes Arabella's grief over losing her parents by showing her what she's destined to accomplish as a result, so it's possible he did something similar here; showing everyone that not only are you coming back, you're coming back without the murderous thoughts that have plagued you the whole time, possibly not wanting the first thing you see upon waking to be your True Companions freaking out.
The Emperor suddenly deciding to join the Netherbrain if you insist on freeing Orpheus, even if you'd sided with him in every other aspect, makes perfect sense when you consider how Orpheus probably feels about the ghaik that abused his powers for untold years; the Emperor knows full well that his death certificate is signed as soon as Orpheus is out of his chains. Even if we could convince Orpheus to let the Emperor live to wield the Netherstones against the Netherbrain, Orpheus would be taking his head as soon as the battle is over. From the cold, calculating, often self-serving mindset of a Mind Flayer, it makes more sense to join the Netherbrain, hope they win, and find some way to escape again in the future; than to take the guaranteed death of staying with the party even to save the world.
Doing some calculations on how Gale could have been abducted by the nautiloïd ship reveal that Gale had left Waterdeep and was headed north. Considering he can also confess he was running out of magic items to feed the orb and was resigned to his fate, it's likely he was embarking on a journey to an isolated place where he could blow himself up without hurting too many people.
When Nettie attempts to stealth-kill the player character, on the logic that it is too dangerous to let them live if they have a chance of turning into a mindflayer, Gale absolutely freaks out. While he's furious on the player character's behalf, he's certainly also aware that if she had succeeded in killing the party she might have unwittingly caused an apocalypse when the orb in his chest went off.
Staking a vampire doesn't kill them, only paralyzes them, as clarified in a conversation with the Gur children. If the player stakes Astarion through the heart, he's still alive—and possibly conscious—when Cazador seizes him for the ritual.
The revelation that ceremorphosis destroys the soul and leaves only a Mindflayer is something that makes it even more horrifying in a setting that has a confirmed afterlife. It is, quite literally, a Fate Worse than Death. We even get implicit confirmation of this with Karlach; if she makes that sacrifice, her manner of speaking is 'very' different in the epilogue, being much more refined and so calm it's almost monotone. A sharp contrast from how she was before. She also becomes much less affectionate towards a romanced Player Character. Mind you, given the Wall of the Faithless, it may still be better than some endings.
The worst possible ending for the Dark Urge only got worse after the 1.5 Patch. When Withers hosts his party for the party, the Dark Urge is the last to show up, and they're a gibbering, barely-sapient mess who can only think about how badly they want to murder their own friends and possibly their lover, their mind having been all but possessed by Bhaal as punishment for their failure. But look closely... notice how the Dark Urge is hunched over, twitching, and one of your dialogue options is to lick your dagger? Notice how the narrator is speaking with a strange syntax, such as combining adjectives and describing murder and violence not with clinical descriptions, but as sensations? Sound familiar? The Dark Urge didn't defeat Orin the Red, they effectively became her, begging the question of if there was ever a time when Orin was at least slightly more sane than we see her in the game.
What happens to Astarion if you give him up to Gandrel is brutal enough by itself—he's tortured to death during interrogation from the Gur, then brought back as a zombie to be fodder in the ritual—but it's even worse when you realize that his soul is needed for the ritual. This means Astarion was forced back into his rotten zombified corpse after death, just so he could be used in the ritual. It's genuinely one of the worst possible endings for any character in the game.
Players have complained of Tav's disgusted and horrified expressions completely contrasting Ascended Astarion's claims that they're the person who trusts him completely. This is a valid point out of universe - the vast majority of players who ascended him did so because they genuinely enjoy interacting with this version of Astarion, and their characters may actually be blindly trusting of and deeply infatuated with him in-game. However, it also tallies with Ascended Astarion's self-centered, megalomaniacal, controlling, abusive behavior. Just because he says Tav trusts him, doesn't mean Tav actually does. It can simultaneously serve as an external threat ("trust me completely or I'll make your life miserable, you have no choice but to trust me wholly and never want to leave me") and internal self-delusion ("I want you to trust me completely so I'll keep telling you - and myself - that").
In Act 2 you can find a pair of rings that allow the wearer of one of them to cast warding bond on the wearer of the other, and their names—True Love's Embrace and True Love's Caress respectively—indicate that they're made to be worn by a romantic couple. Pretty cute, until you read the diary left on the skeleton holding one of the rings. Turns out a Cleric of Shar proposed to her boyfriend with the ring meant to be held by the recipient of the redirected damage before going into battle, and tried to convince him his nosebleeds and dizzy spells were just stress when she returned from soloing an entire troop of Harpers without a scratch. Basically, she tricked him into taking all the damage for her and then gaslit him when he started to realize something was up; a small-scale version of Ketheric and the Nightsong. To make matters worse, when he finally goes to the House of Healing, the nurse keeps asking about the ring; and when he finally answers, the nurse tells him to leave and not come back; given that high-ranking Shar followers are encouraged to spurn romantic relationships in favor of giving their whole hearts to Shar, it's not impossible that this Cleric, and the nurses at the House of Healing (who, by this point, had been instructed to only treat Shar followers), only saw this poor man, who loved this woman with all his heart, as a damage sponge.
Astarion occasionally makes disparaging remarks about poetry and/or poets, which on the surface just seems like him cracking a totally harmless, meaningless joke. But if you pay close enough attention, he'll actually mention in passing that Cazador is a fan of poetry. If you have romanced him, he further reveals that he believes (and has believed for decades, if not centuries) that the runes carved into his back were a poem. No wonder the poor guy hates poetry so much.
In the Withers party epilogue, one possible letter from Elminster reveals he met Gale when the latter was 8 years old, and comments on how Gale Used to Be a Sweet Kid who cried over accidentally destroying plants with his magic. This casts new light on Elminster in Act 1, who was clearly reluctant to deliver Mystra's order for for Gale to suicide bomb the Absolute. Bad enough to have to deliver such news to a friend or a colleague, but to tell someone you've watched grow up from childhood that they've been ordered to their death is downright tragic.
If you romanced Astarion and bring him to the Drow Twins after dealing with Cazador, he'll enthusiastically agree to it; saying that he wants to experience things like that for himself now that he's free to explore his own desires, and promises he'll bail if he doesn't enjoy it. However, the narration will still say that he looks like he's a million realms away—the same text if you bring a non-romanced Astarion along. This has two interpretations, and neither are good. One possibility is that he still struggles with saying no, and feels like he owes you for helping him deal with Cazador, and that's the only reason he agrees to it. A worse possibility, albeit more likely given that he reacts to the foursome suggestion before you do, is that he's jumping at the chance to experience sex on his terms to prove to himself that things are all better now that Cazador is out of the picture, only to be forced to face the reality that 200 years of sexual trauma did not die with Cazador, and he has a long way to go still before he's ready for stuff like this.
Ascended Astarion "boldly" throws himself into the center of the group to be worshipped (something he possibly had little of in his past as an abuse victim), but afterwards the player character notes that "there's no joy in his eyes, only an intensity that makes [them] shiver". The first possibility is that he had dissociated during it, proving that his trauma remains even though he thought finally killing his abuser and Ascending was supposed to fix all of that. Alternatively, Ascended Astarion may instead simply be experiencing a curse: there is no happiness for a true vampire. He can try to do anything and everything, pretend all he likes, but he is incapable of ever truly feeling happy again.
In some idle chatter, Gale asks Astarion if he’s ever considered worshipping a god, which Astarion replies he prayed to them all and no one answered. This unfortunately makes sense as most if not all good and neutral aligned deities despise the undead and probably wouldn't be interested in saving a vampire spawn, while evil aligned gods would be more interested in helping Cazador/revel in his and the other spawn’s suffering.
Astarion, pre-character development (depending on the run) consistently approves of cruel actions. It's because cruelty is all he's known for two centuries; the torment he went through had thoroughly convinced him that cruelty was the rule of the world, and anyone who isn't cruel would not survive.
While Astarion freaking out if the player chooses to joke about him having a mole or sagging skin is a funny poke at vanity (Tav smirks and slightly squints their eyes), he then genuinely looks upset and ends the conversation with an "Excuse me." Firstly, he's a Proud Beauty who sees his lack of reflection as another thing Cazador took from him. Secondly, he's spent two centuries being forced to use that beauty and charm as a tool, and most likely has devolved to seeing his looks/body/allure as a major, if not the sole, determinant of his worth. Depending on player choices, in a later scene, he may also snap that he supposes there's not much point to him if he won't have sex, and that it's his only talent.
reveal that Gale had left Waterdeep and was headed north. Considering he can also confess he was running out of magic items to feed the orb and was resigned to his fate, it's likely he was embarking on a journey to an isolated place where he could blow himself up without hurting too many people.