We can all be thankful that the exorcism didn't work; the monsters stayed monstrous, and eventually made their way into his films: The Devil's Backbone, Cronos, and two Hellboy features. In his early student films, Del Toro did his own makeup and prosthetics, and those who've worked with him have been appreciative of his understanding. He doesn't, as do many other directors, go straight to CGI, but prefers to do whatever he can with makeup and prosthetics, then "tune them up" with just enough CGI to make them seamless. It's an advantage to the actors, too, who get to add their bodily gestures, and to those who act with them, who don't have to pretend to see things in a greenscreen room.
But the real essence of Del Toro's genius is that he believes in fairy tales, and respects them. He understands their arcance and ancient logic, and knows there should be three fairies, three doors, and three special tasks. And most of all, he believes in his actors, eliciting astounding performances from them, especially Ivana Baquero, who plays Ofelia. He knows that a film such as this must create a world, and that the world -- or worlds -- must be internally consistent. And yet, with that foundation, he lest his fancies fly, and they travel much further, and deeper, than any other fantastical films of our era.