Chara will feel their breathing and pulse thundering against their arm. Sans will probably hear their breathing, in fact, since every exhale sounds ragged and growling.

This shouldn't be happening why is he here this needs to stop they can fix it fix it how can they fix it there has to be a way but Sans' words are distracting and the knife is distracting and their own head is filled with foggy rage and fear (the latter not for Frisk).

Their lips draw back, teeth bared. Their eyes aren't on Sans, more staring coldly in a middle point between him and where they can feel Chara's hand. "Stop..." It isn't clear who they're talking to, voice small and weak and not matching their expression or emotion in the least.

Determination.

Except their determination's just leaving them with their thoughts spinning out of control.