Miss mime is trying. While her face is rock-steady at this point, her hands are clenching and unclenching. Trying. Trying to make it work. If she wasn't a mime, this sort of subtle copying would be nearly impossible. He has years of experience. She doesn't. It's almost a miracle that she can actually read his expressions at all.
Erin's face is actually kind of scary to watch. Because as he comments on her doing on piece at a time? Instead of focusing on just trying to mirror the look, she focuses on single muscles. First her eyebrows moving into place. Then her mouth. Different sections of her face moving, looking almost machinelike as she tries to perfect what he's doing right now.
The mime looks over at him for a moment before lifting a hand and holding it level to the earth, then simply tilting it left and right. There's a smile. Then a frown. An ecstatic grin. A teary-eyed face. There's gritting of the teeth, then a relaxing look. Then she simply shrugs.