i always find it interesting when fanfic writes papyrus as very emotionally aware and empathetic and sans as nervous and unable to read others' intentions because canonically, it's the other way around.

sans can literally figure out exactly how many times he's killed you by the look on your face. meanwhile, papyrus mistakes your clear disgust with his spaghetti for passion and projects his own feelings about puzzles and friendship onto you.

the difference is that papyrus is extraordinarily COMPASSIONATE, even if he's not super empathetic. he can't always tell what someone is feeling, but he cares deeply and tries to help as soon as he knows that something is wrong. he is also usually pretty good at figuring out what people need! like, for instance, knowing alphys needs a friend in the true lab, or realizing undyne needs a challenge to motivate her to become friends with you.

meanwhile, sans is so depressed and apathetic that he just... doesn't seem able to muster up enough energy to care for other people in the same way. he understands how other people are feeling, but that doesn't necessarily translate to compassion for him. he cares about his brother, and he cares about toriel, and he eventually comes to care about you in a pacifist run but... other than that it seems like he struggles to care about others.

anyway, i just find it interesting that people mistake papyrus's compassion for empathy, and sans' apathy as a lack of empathy.