Napstablook making a top hat out of his own tears which he calls, "Dapperblook."
You can call Toriel on the cell phone and choose from a few options of what you want to say to her. If you choose the "Call her 'mom'" option and then choose the "Flirt" option, she will take note of it and call you an "interesting child."
When examining the cactus in Toriel's room:
Ah, the cactus. Truly the most Tsundere of plants.
If you fight the dummy at the beginning of the game, and intentionally miss each time, eventually it gets tired of your "aimless shenanigans" and just floats away, ending the battle.
In your very first real battle against a froggit, choosing compliment ends up revealing that it is scared... Of Toriel whom slides on screen and glares at it automatically winning the fight for you.
On the other hand, if you choose threaten, it will be flattered.
Flowey's increasing rage when you don't fall for his first trap is hilarious, especially if you're doing it in revenge for him chiding you on a previous game.
The music also distorts each time you avoid the "friendliness pellets", highlight how things are going off script. Flowy is not happy.
Eventually, if you keep it up he completely loses it, reveals his Nightmare Face, and attempts to insta-kill you...at which point the bullets cancel each other out, giving him a couple seconds to go Oh Crap before he gets a fireball to the face via Toriel.
Meta example. Toby's rules for the NPC reward tier has him recommending that backers create a snail NPC because they are actually monsters in real life.
The enemy mentioned under Peer Pressure Makes You Evil has "bullets" that are actually a little animated bug doing an armwave dance once you get rid of its allies. It stays at the bottom of the screen and makes no effort whatsoever to attack your soul.