Sarah's attention has been completely on the screen, wide-eyed. Sure, they had the giant viewscreens and such back on Battleworld, and some of the folks from that Suburb in Denver had some videos that Peter was able to rig to play on said giant screens... but somehow its not the same. Those were obviously monitoring stations, meant for business, this is obviously meant for dragging the viewer into the movie.

And then there's Gretchen. She's not been quite sure what to make of the butterflies in her stomach, or the anticipation that somehow wasn't just going to see a movie.

The combination of invulnerability and being lost in the movie being what it is, at first she didn't even notice the pinky touching hers. By the time she did, and realized it had been there a few moments, it seemed like it would be more awkward to flinch, or any of her first reactions. So without looking, not daring to look, she leaves her hand where it is, still touching Gretchen's.