River is giving him a 'oh, come ON now, you can't just stop there' look.

And then her eyes flick to Aziraphael's knees -- his trousers are black, and in perfect repair, if a little rumpled -- and she looks briefly astonished. It fades into confusion as she glances back up at Aziraphael.

And then, the confusion is gone, and she's starting to giggle.