An interesting interview with Guillermo del Toro (Pan's Labyrinth) at the Writer's Guild of America.

POSSIBLE SPOILERS

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"What I was trying to do was find a magical universe that was left out in the rain for too long. The way I see the faun and fairies, they've been down there in the mossy pit for so long that when the girl finally shows up, they are a little insane. The faun is all twitchy and old and full of quirks and the fairies are eating meat and scratching their asses. They're like dirty little monkeys that don't speak, but as the movie progresses, the fairies become more and more human and the faun becomes more and more elegant and beautiful.

The idea was that, although I didn't want to make a Syd Field/Robert McKee plot point in the screenplay, as the girl went through the tests, the magical universe around her started healing itself." -- Guillermo del Toro