It's scary, scarier for River because she's figured it out and maybe we haven't yet, but Stonehenge is a real place and this time is a real time, so maybe Amy's like a memory magnet pulling these things into alignment, except then you've got the Pandorica, of which the Doctor has remained steadfast about the existence until today, and who knows what else. So then is Amy creating these things out of thin air? Are we in the Matrix? Whatever it is, River is right to be scared.
Rory comes out to see if Amy's okay, and gets all territorial about the fact that one of the Romans gave her a blanket, and she is put off by the weirdness, but something about him -- something sad, weary --- makes her ask his name, about which she laughs: "It's just not what you expect Romans to be called. What's it short for, Roranicus?"
She doesn't know it but she's crying.
River calls the Doctor and -- don't ever do this, it's terrifying when you do this -- tells him not to raise his voice or look alarmed: Just listen. And the music -- and maybe that parallel act earlier, with the horseback riding -- lets you know these two things, River and the Doctor inside, Amy and Rory inside, are about to become one thing. And it's shivery, and it's sad, and it's scary, and it's plotted out so well that your growing sense of dread outpaces the growing sense of dread for everybody else, even scared-to-death River and worked-to-death Doctor, because you can see it lining up, and the ways it could be variously bad or sad or both or terrifyingly both.
Amy is crying, but not because she's sad: It's because, for reasons she doesn't understand, she is happy. This is scary for her, scarier for us, but not scary at all for Rory. It's terribly sad, for Rory, but he doesn't know it yet; as much as I'm unsure about Rory's heroicness when all is said and done, I must say that this is the worst thing to happen to anybody in this entire season. It's almost too gruesome to think about, if you think about it.
Because meanwhile, as Amy is stretching herself out into these unknown, time-breaking places where she's happy because she doesn't know why, because she has accomplished the Doctor's miracle, this is what River is saying: "They're all right here in the storybook: Those actual Romans, the ones I sent you, the ones you're with right now. They're all in a book, in Amy's house, a children's picture book."
Because something -- not Amy's specialness, not the magic of the TARDIS, not anything like a miracle -- is using Amy, cruelly, all the pieces of her childhood, all the dearest things of her -- her favorite things, the things she loves most, the things she loves so fiercely she still loves them when they've vanished -- in order to create this trap. The Romans, the Pandorica. So perfectly well that even the lipstick worked on them. So perfectly well they don't even know that they're fake. So perfectly well that a joy rises up in her that is so fierce and full of love it scares her.