Guillermo del Toro is famous for compiling books full of notes and drawings about his ideas before turning them into films, something he regards as essential to the process. He left years worth of notes for this film in the back of a cab, and when he discovered them missing, he thought it was the end of the project. However, the cab driver found them and, realizing their importance, tracked him down and returned them at great personal difficulty and expense. Del Toro was convinced that this was a blessing and it made him ever more determined to complete the film.
Guillermo del Toro repeatedly refused offers from Hollywood producers, in spite of being offered double the budget provided the film was made in English. He didn't want any compromise in the storyline to suit the "market needs".
Stephen King attended a screening of the film and sat next to Guillermo del Toro. According to Del Toro, King squirmed when the Pale Man chased Ofelia. Del Toro compared the experience of seeing King's reaction to winning an Oscar, which he eventually did with The Shape of Water.
Doug Jones had to memorize not only his own lines in Spanish, a language he does not speak, but also Ivana Baquero's lines, so he knew when to speak his next line. The servos in the head piece that made the facial expressions and ears move were so loud, he often couldn't hear her speak her lines.
After the first week, movie theaters in Mexico placed signs on posters warning about the movie's graphic violence, because so many people brought small children to see it. It was also reported in the news in Spain.
In Spanish, when addressing two or more women, one would say "Bienvenidas." The presence of any man or boy would require the use of "Bienvenidos." When Captain Vidal welcomes Ofelia and her pregnant mother, he says "Bienvenidos," showing that he's most interested in his unborn son.
The faun in the movie was inspired by a lucid dream Guillermo del Toro repeatedly had when he was a child: every midnight, he would wake up, and a faun would gradually step out from behind a grandfather clock.
On disc 2 of the Platinum Series DVD, Guillermo del Toro points out that he intentionally placed "Faun" references throughout the movie. The most obvious examples are at the entrance to the labyrinth and in the shape of the Giant Toad's tree.
Doug Jones stated on disc 2 of the DVD that the Pan suit was the most comfortable, and well made suit he had ever been cast to wear. Thanks largely to the suit being divided into many sections. Having the legs anchor to his hips and not his shoulders distributed the weight better, and having the stomach section separate from the shoulder section gave him better range of motion.
Although Doug Jones plays El Fauno and The Pale Man, he doesn't voice either character. Doug Jones spent his 5 hours in the makeup chair practicing Spanish to play the role. In the end, Del Toro hired Pablo Adán, a theatrical actor, to voice El Fauno. Jones's efforts were not in vain though, as it made Adán's job of synchronising with his lips and Ivana Baquero's job of interacting with the character easier.
The ruined town seen during the opening sequence of the film is the old town of Belchite Zaragoza, in Aragón, Spain. It was also used by Terry Gilliam for The Adventures of Baron Munchausen (1988). The town really was destroyed during the Spanish Civil War and never rebuilt.
According to the director, the scene involving the giant frog was going to be shot in an extravagant dome "tree" set. Three days prior to shooting, he realized that the frog wouldn't seem so giant in the massive set. The tree tunnel set was constructed in 2 days.
The film's original Spanish title is "The Labyrinth of the Faun." It was retitled "Pan's Labyrinth," after the goat-human nature god Pan, in English-speaking countries, German-speaking countries, Scandinavia, The Netherlands, and Croatia. Guillermo del Toro said that he felt the Faun is just a normal faun and that Pan himself was too dark and sexual for a fairytale starring an eight-year-old girl.
Proposed as the middle film in a trilogy about the Spanish Civil War. This would make The Devil's Backbone (2001) the first film in the series. As of 2016, Guillermo del Toro has no immediate plans (or indeed time in his schedule) to start work on the third film.
Sergi López, who plays Captain Vidal, was considered a melodramatic or comedic actor, and the Madrid-based producers told Guillermo del Toro: "You should be very careful because you don't know about these things because you're Mexican, but this guy is not going to be able to deliver the performance". Del Toro replied "Well, it's not that I don't know, it's that I don't care".
Also on the supplementary disc of the Platinum Series DVD, Guillermo del Toro indicates that the film is quilted with a pattern of threes. He mentions that this was intentionally done so as to evoke a greater sense of fairy tales and mythological traditions, both of which typically feature a hero or heroine existing among threes; for instance, "the three tasks".
In an interview, Guillermo del Toro hinted that the nameless soldiers who die in the woods, including the one shot through the hand by Vidal, are the surviving children of the orphanage in "The Devil's Backbone" (2001). This confirms that idea that most of Del Toro films happen in the same universe.
Note the set design. When Ofelia is the prominent character in a scene, the design is circular, to emphasize her embracing of the fantasy world. The real world scenes sport a much more straight, angular design.
Sergi López met with Guillermo del Toro who played out his vision and story for the film. Highly impressed, Lopez asked for the script, to which del Toro replied that he hadn't written it yet. Lopez had to wait another year before the completed script arrived.
The trivia items below may give away important plot points.
Although audiences have interpreted the film's bittersweet ending as everything from a religious metaphor to a psychological allegory, Guillermo del Toro offers a simpler, but more poetic, explanation, "I always think of that beautiful quote by Søren Kierkegaard that says the tyrant's reign ends with his death, but the martyr's reign starts with his death. I think that is the essence of the movie; it's about living forever by choosing how you die."
While some viewers believe Ofelia's eating the grapes in the Pale Man's den to be something of a "too dumb to live" moment for the young heroine, it would actually seem to be a reference to what turns out to be her ultimate virtue: Courageous disobedience. According to Guillermo del Toro this theme is why the movie is set against the backdrop of falangist Spain (where disobeying the fascist regime was dangerous), and the final test of character for the princess confirms the importance of disobedience as well.
Of course, Ofelia hadn't eaten for a day, and was likely very hungry, which probably didn't help.
When Ofelia is rejected by the Faun for not following the rules in the book with the second task, he claims that there were other reincarnations of Princess Moanna before her, but like her, they all failed. It's entirely possible that Mercedes was originally one of the tested reincarnations of Moanna.