Chara seems unsurprised by Frisk's disgusted gesture- in fact they don't really react until Frisk addresses them directly. Only then does Chara slowly lift their head to look the other child in the eye.

Chara looks... tired, more than anything else, though their mouth tightens into a thin line and they give Frisk a warning shake of the head. Then, after a quick glance at the door- it's a very solid looking wooden one, and firmly shut- they tap one finger against their lips in a very clear "Shh" gesture.