Interacting with people on a healthy, normal social level is more complicated than it looks, and she's just glad that Reiner's not going to be difficult about it. She doesn't want to deal with it, not normally and especially not in territory that an enemy could appear at any moment. They could afford to talk, at least, to look like normal humans so long as those few enemies don't show their faces. They had to if they wanted to fit in.

(Never mind that she'd just thrown someone to the ground that was more than a foot taller than her.)

Annie's particularly practiced at watching their surroundings without being suspicious about it, so long as no one did strike her wrong. By all rights, she looks utterly disinterested with everything around her.