There are photos that you just can't stop looking, and this is one of them. Cartier-Bresson spoke, a long time ago, about the "the decisive moment", and this it.
In the first place, the expression of the girl; she doesn't really seem troubled with the hand on her breast, it seems more like she is explaining something to the author of the photo, something like "Oh, please, don't make more photos...". But you can't see that she is not herself at 100%.
At the back of the photo, there is Prince William, the future king of the United Kingdom. He is definitively drunk, worsened by the fact that he has the typical expression of a drunk people trying (worthless) not to seem drunk. He puts all his effort to have a seriour pose, but everything in his face and body is beatraying him.
And at the right we have Prince Harry. We don't know if the is really drunk or not (by the colors of his face, he may be), but what is sure is that he doesn't have time to loose.
