Their expression's rather muted. There's some anger there, some hurt, some disappointment, but that's not all they're there for. They aren't finished talking.

Frisk starts to answer, opening their mouth--

And then Sans is there.

They jerk around halfway so they can see him and keep an eye on Chara at the same time. "What--? Sans?" Surprise is the only thing in their voice, now, since they were supposed to be alone--it'd been like the entire world was only the two of them. Sans shouldn't be here.

...Sans very much shouldn't be here, they register, slowly. Too slowly.