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The utterly exasperated expression Temmie's Nightmare Face makes when Asgore approaches.
If you use the Cell Phone under the Items menu, Chara randomly dials other people than Asgore (although they can accidentally call him too). This includes Sans picking up and reverse-prank calling Chara.
After finishing the training, it turns out that the Dummy was in fact the Mad Dummy, complete with "Dummy!" in the style of the Menacing katakana. After it's rant, it tries to throw a knife at Chara. Cue it missing completely, and then flying off. It then ends with a dumbstruck Asgore, who saw the whole thing, calls it "unexpected".
Chara's dismayed expression when they fall into the sewer, with their hat (if you equipped it) floating gently down after them. When they pop back out of the water, they're drenched head to toe. Climbing out of the sewer has them shaking themself dry like a dog, even in Ruthless.
One of the small bits of comedy while on a Ruthless route is Chara describing the bitter tea as "Not my cup of tea" when checking it from the item menu.
Also, even on a Ruthless route, Chara is dumbfounded and disgusted by Asgore's fridge being filled with snails.
Bugerpant in general. Even when he's not working for Mettaton, he's still a Burger Fool with loads of hilarious dialogue options and faces.
When Chara suggests proposing to Flara, his reaction to explain love before revealing that he'd been rejected many times before.
Bugerpant: (oh god how do i explain this to the kid)
Right after Bugerpant tries explaining to Chara how proposing isn't a good idea, the "PROPOSE!" dialogue option doubles down and becomes an even more enthusiastic "PROPOSE!!!"
"Bugerpant"'s name, which he is contractually obligated to introduce himself as. Say it out loud. Didn't think it could get worse than "Burgerpants", did you?
Burgerpant's incident with Mettalot is detailed in a cutscene that can be viewed in the extras menu or on YouTube titled: "Burgerpant: Origins". The selling moment has to be the super smug expression that Flara makes towards the Greasers once she tells Burgerpant that he could get a shot at becoming an actor if he attends Mettalot's show. For bonus points, her face matches Burgerpant's current dialogue portrait.
Flowey's reaction to the second encounter with the Temmies is simply "Well, that sure happened!"
If you replay the first section multiple times, Temmie gets more and more iritated with you during both of the first two encounters.
The ending of the first demo. Papyrus gets upset that his appearance gets stunted by Sans. Sans then says that the "build-up" will be worth the wait, having Papyrus agree.
Papyrus and Sans—we mean, Crossbones—are just as funny as expected, if not more:
Normally, Sans will surprise Chara by appearing right behind them as he declares them "the bad guy." However, if you've already seen this cutscene, Chara will instead turn around before Sans gets a chance to get his lines out, catching him off guard and causing him to forget his lines.
When Papyrus demonstrates how to use the pink goo, he accidentally crashes into a wall and makes a Papyrus-shaped hole, before using the goo again and being sent flying and crashing into a tree.
There's a room where you find a lamp that is shaped like Crossbones. As you continue to walk, it appears to talk to you, implying that Crossbones has disguised himself as the lamp or is hiding behind it, only for him to come onscreenfrom the right.
Crossbones makes you race against him on a board that resembles a dog (similar to how the River Person's boat in Undertale sometimes resembles a dog), while he rides the "Crossmobile", which is just a small tricycle. As you reach the end, Crossbones ends up crashing into Papyrus, who was painting a picture, and the picture somehow ends up displaying a shot of the two skeletons crashing into each other.
If you lose the race, Crossbones will tell you that you sucked, and Papyrus questions why he would say something like that instead of something more encouraging. After the two leave, checking Papyrus' painting, which is positioned so that the player can't see it, will have Chara say that it's a picture of their failure, and that they're glad no one else can see it at that angle.
After entering the Royal Navy zone, where Crossbones isn't allowed, there's a room where you can see him standing with Papyrus, but it's actually just a cardboard cutout. You'd think that Papyrus is one too, but no, it's actually the real Papyrus.
Reaching the room with the binoculars will have Crossbones in a disguise... that consists of his civilian outfit and a fake moustache, but still wearing his mask. Chara seems to recognise him as the skeleton salesman they encountered earlier, but not as Crossbones, as they add the disguise features to the "Skeleton" entry in their journal.
The area where you fight Crossbones bears resemblance to the boss rooms from Deltarune, right down to having a stray pixel nearby.
The fact that the skeleton's house is almost completely the same as in Undertale... Except the perspective is vertically flipped.
While in Sans' house, he'll offer a quest for you to change the channel for him since he can't find the remote and doesn't want to get up. Once you do so, he'll give you 1G... only to immediately take it back as the channel changes back, so the quest reopens. This happens several times in a row. Eventually, Chara gets so fed up with this that they put the 1G in their mouth so that Sans won't take it back, only to be disgusted with themselves afterward.
Most of the channels mention that Sans is or has been break dancing (which he says is due to a coincidence where the shows happened to be filming in areas where he likes to do so)... Except, ironically, the dancing channel.
Drinking the Spider Latte while fighting Muffet on a Ruthless route has Chara describe it as "Bitter, probably just like her personality."
Repeatedly talking to Flowey while at one of the puzzles on a Ruthless run has him get increasingly exasperated, repeatedly pointing out that he can't solve it due to his lack of arms and legs, before eventually just leaving.
Mad Dummy returns, this time possessing a doghouse. Checking the doghouse will have Chara say that it's just "a cute doghouse", but it will follow them as they walk before revealing itself with the same musical sting from when revealing itself in Ruined Home.
Checking it after it moves but before the ghost reveals themself has them assume that "it must be a mobile home".
Koffin-K and his henchmen, Harry and Larry, are also likely to make you laugh:
How do Harry and Larry get around quickly? They have helicopter blades in their hats.
There are various points where you can't go to certain areas because the path is blocked by a float with what appear to be badly-made models of the three on it, which plays music that starts slowing down to show that it's not working properly. Chara repeatedly comments on how nightmarish they find it.
When Koffin-K decides to make Chara into his servant, he makes them wear a different outfit. There are three different ones; a regular butler suit, a suit with broad shoulders and a white neck ruff so high that it covers part of their face, and an oversized robe that Koffin-K says was once his. What's funnier is that Chara's face has angry eyebrows if they're wearing the oversized outfit, implying that they're annoyed at being forced into something way too big for them.
On the way to the festival, Chara and Koffin-K come across a piano. Interacting with it multiple times has Chara play one key at a time, repeating every four notes. It's the first 4 notes of Megalovania.
On a Ruthless or Evacuation route, Koffin-K keeps failing at his attempts to change Chara's outfit, and decides to leave them alone due to them being scarier than he is. After this, Flowey remarks that they almost became his servant and would have had to do boring chores for him, though he tells Chara "Don't ask how I know that". Talking to him again has him Sweat Drop while saying "Seriously, don't".
The battle with Harry and Larry results in the two of them starting to fight each other and turning into a Big Ball of Violence that rolls out of the mansion, while taking Chara with them.
After the battle, when Koffin-K tells Harry and Larry that he values them equally, the two end up crying puddles of tears that are then wiped up by the nearby Woshua-like monster.
In Sans's hangout, Koffin-K doesn't seem to realise Sans is his arch-nemesis Crossbones, despite Sans being in Crossbones' punishment shack and talking about being Crossbones. Sans almost spills the beans to him... before deciding it's funnier to not tell him.
Talking to Koffin-K enough times during the hangout has him summon one of his bats to carry him off, saying, "IT'S KOFFIN TIME!" as it does... only for Sans to immediately grab him and put him back.
If you decide to leave the hangout area and enter it again, a sudden tape appears on the left. If you inspect it, Sans and Koffin-K have this to say...
Sans: whoa. where'd that tape come from?
Sans: isn't that the last surviving copy of the cult classic "koffius?"
Checking the camera at the festival while on a Ruthless route will have Chara take the time to take a selfie of themselves.
Checking Sans' coat hanger to get a closer look at it reveals that the topper is a super-detailed version of Crossbones' head making a really creepy smile. After that, Chara refuses to take another look at it.
In the Ruthless route during the Navy Chase Sequence, if you decide to go left and interact with Sgt. Splosion after believing Chara's about to escape, it prompts this exchange:
