In the shooting script, River said something like, "No, dummy, I'm not a multiple." But her facial expression expressed that same idea better, and with no words -- at least, for neurotypical members of the audience.

Her facial expression expressed the following things:
"Your suggestion is not true."
"You're being stupid for even thinking of that."
"I care about you, and trust you, because otherwise, I wouldn't be making this face."

It was the sort of face that a sister might make to a brother, if they cared about each other, and teased each other in a friendly way.

So it did a lot of things very well -- but only for people in the audience who can read faces.