Chara flinches at the sudden movement, even if it's just of Frisk's head. When nothing happens they exhale, very quietly, and then reach over to touch Frisk's arm. Once they have the other child's attention they lean over and whisper some advice:

"Pretend you're a doll."

Chara then lowers their arm and leans back in their chair, mumbling, "It goes easier if you do."

A clacking noise, like shoes stepping over a hard surface, can suddenly be heard from behind the door, as if someone's puttering about.