Guillermo del Toro repeatedly said "no" to 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".
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.
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.
Doug Jones had to memorize not only his own lines in Spanish (a language he does not speak) but also Ivana Baquero's (Ofelia) 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 couldn't hear her speak her lines.
After the first week, because so many parents brought small children to the film, movie theaters in Mexico placed signs on the posters warning about graphic violence. It was also reported in the news in Spain.
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 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 was destroyed during the Spanish Civil War and never rebuilt.
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 being seen at the entrance to the labyrinth and in the shape of the tree of the Giant Toad.
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. It may also be a reference to the Narnia story "Voyage of the Dawn Treader" in which the protagonists encounter en enchanted feast which remains fresh, delicious and tempting forever, but from which one should not eat. Finally, of course, Ofelia hadn't eaten for a day, and was likely very hungry, which probably didn't help.
Guillermo del Toro said that he felt the character of Pan was too dark and sexual to play in a film opposite an eight-year-old girl. The film is only called "Pan's Labyrinth" in America, other English-speaking countries, German-speaking countries, The Netherlands, Scandinavia, and Croatia; everywhere else it's called "The Labyrinth of the Faun."
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 in large part 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.
Also on the supplementary disc of the Platinum Series DVD, Guillermo del Toro indicates that the film is quilted with a pattern of three's. 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 amongst three's; for instance, "the three tasks".
The movie is set in a verdant forest landscape but in reality it was shot in a location that had experienced the worst drought in years. For that reason, any bullets or explosions had to be put in digitally so as to reduce the risk of fire.
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.
The trivia items below may give away important plot points.
And 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."