Well, first you have to get the character's voice. Sans from 'Undertale' has this incredibly specific cadence and a type of humor that's all deadpan delivery, puns, and this weird meta-awareness of the game's mechanics. The reader fics that really nail the humor don't just have him cracking jokes—they embed his wit into the narrative structure. He'll make a pun about the fourth wall, or comment on how the 'save points' are acting up, turning a romantic or dramatic scene into something unexpectedly funny without undercutting the mood entirely.
I think the real cleverness comes from how writers balance his laziness with surprising moments of insight. He might be too tired to go on a proper date, so the whole thing becomes a pun-filled grillby's visit or a stargazing session where he points out constellations he's named after bad jokes. The humor isn't just slapstick; it's a conversational game where the 'reader' character has to volley back, and the wit lies in that back-and-forth. A lot of stories fail because they make Sans a joke machine and the reader just passively laughs, but the good ones make the interaction feel like two sharp minds playing off each other, even if one is a skeleton who'd rather be napping.
What's interesting is how this humor often serves as a mask for deeper stuff, which is very in-character. The witty deflections, the puns to avoid a serious talk—it creates a unique romantic tension that's more about what's not said with a straight face. You're laughing along, and then he drops a single, sincere line that hits harder because of all the comedy surrounding it. That contrast is where a lot of the emotional payoff lives in these stories.