* AbilityOverAppearance: Ofelia was written to be a younger girl, aged seven to nine. However del Toro was so impressed with Ivana Baquero (who was twelve), he rewrote Ofelia's age to accommodate her.
* ActingForTwo: Doug Jones plays both the Faun and the Pale Man.
* BigNameFan: {{Music/Bjork}} and Creator/StephenKing. The former was inspired to write the song 'Pneumonia' after seeing the film. The latter sat beside del Toro at a screening and squirmed during the Pale Man scene. Del Toro compared his reaction to that with winning an Oscar.
* DoingItForTheArt: Guillermo del Toro gave up his director's salary to see this film realised.
* FatalMethodActing: Narrowly averted. Actor Manolo Solo was nearly killed when horses fell on top of him.
* ImageSource: This film provides the page image for:
** EyesDoNotBelongThere
** SchmuckBanquet
* PlayingAgainstType: Much of the cast. Creator/SergiLopez (Captain Vidal) typically played slimy, arrogant creeps who were somewhat LaughablyEvil. Creator/AlexAngulo (Doctor Ferreiro) had been considered a lightweight comedic actor. Creator/MaribelVerdu (Mercedes) had played mostly love interests, and her most famous role (at least in the English-speaking world) had been Luisa Cortés in ''Film/YTuMamaTambien''.
* WhatCouldHaveBeen:
** The first outline of the story was about an adult woman that found a hidden labyrinth behind an old house's bookcase. She would then be ''raped'' by a faun living inside the labyrinth, and as a result she would give birth to a magic-powered baby. Creator/GuillermoDelToro is ''that'' weird.
** An earlier draft of the script had the situation more ambiguous as to whether the magic was real or not (as Ofelia would encounter a dragon that speaks with Vidal's voice). In the finished film, it's strongly implied that the magic is definitely real.
** This plot idea incidentally bears some resemblance to Creator/ArthurMachen's ''Literature/TheGreatGodPan''. The movie that happened ends up taking more influence from a different Machen story, ''Literature/TheWhitePeople'', which features a little girl associating with dangerous fairies.
** The Pale Man wasn't originally in the movie. The scene was originally meant to be Ofelia being tasked to retrieve a diamond from a sleeping dragon's hoard. The dragon would wake up when she removed a goblet from the hoard. It would then chase her, whilst ''speaking with Vidal's voice''.
** Del Toro's ''Cabinet of Curiosities'' (an art book that collects some of the director's sketches, complete with English translations) shows that one of the earlier incarnations of the Pale Man was a wooden doll in a tree.
** The toad was originally going to be a more active adversary, leaping around in a large, womb-like cavern and attacking Ophelia. Unfortunately the animatronic model ended up not being up to the task even with the aid of CGI, so it was reduced to the lethargic beast in a narrow tunnel that we see in the film. Additionally del Toro realised that the toad wouldn't seem as scary in such a large set, so the tunnel was considered a better choice.
* WordOfGod:
** Some fans have interpreted the magic elements to be just Ofelia's imagination, and the ending to be [[spoiler: a DyingDream]]. Del Toro has claimed that he considers the magic real, but has left it open to interpretation.
** He's also claimed that Ofelia's famous IdiotBall moment of eating food from the Pale Man's table to be a representation of her biggest virtue: courageous disobedience. According to him, it's a sign that she doesn't blindly follow orders - which emphasises her as a {{Foil}} to Vidal.
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