Upon meeting to discuss the movie, Hellboy Creator Mike Mignola, and Writer and Director Guillermo del Toro, decided to reveal to each other their choice for the lead role of Hellboy. They both said at the same time, Ron Perlman.
Very early on in pre-production, the filmmakers considered putting Hellboy's giant stone hand on his left arm rather than the right, so as to grant the actor playing Hellboy the full use of his right hand. Ron Perlman turned out to be left-handed, so the filmmakers were able to preserve the Right Hand of Doom.
The filmmakers decided not to make the film too bloody, so as to avoid an R-rating, and thought of things they could substitute for the blood. While filming the fight between Hellboy and the monster in front of the people in the subway, Ron Perlman suggested that Hellboy could grab a gumball machine and start beating the monster over the head with it, with the flying gumballs standing in for the blood. Instead, Hellboy rips a payphone off the wall and beats the monster over the head with the phone, with the coins that go flying out standing in for the blood.
In 2012, Ron Perlman once again endured the four-hour make-up routine required to transform him into Hellboy. Not for a sequel, or other acting job, but to fulfill the Make-A-Wish request of a six-year-old boy named Zachary who had leukemia. Creature effects house Spectral Motion applied Perlman's Hellboy make-up (and later, also made up Zachary as Hellboy as well), and then Zachary got to spend the day hanging out with "Hellboy".
Much of the demonology in the film is inspired by the Cthulhu Mythos developed by H.P. Lovecraft, a horror writer in the 1930s. The Sammael creatures have characteristics of both Nyarlathotep and Cthulhu. Elder gods, many eyed and tentacled, sleeping at the edge of the universe, are a staple of his books.
When Guillermo del Toro met with producers, many changes to the Hellboy character were suggested. One idea was to have Hellboy be a human who transforms into Hellboy when he gets angry. Another suggestion was that he came from Hell, but was a normal human. del Toro vetoed all such attempts to alter the character.
It took anywhere from five to seven hours to apply Doug Jones' make-up. It took another three hours to reverse the process. Sometimes, some of the pieces were not removed, so that it would be easier to put the costume on the next day.
Guillermo del Toro for years considered this film a dream project, and had always wanted to cast Ron Perlman in the lead, but could never secure a budget or studio approval. After the massive success of Blade II (2002), del Toro was offered Blade: Trinity (2004) or Hellboy, and though he briefly considered trying to schedule both in, he chose Hellboy.
The tombstone we see right after Hellboy enters the Russian cemetery was digitally altered. The name on it was changed to Mike Mignola's in Russian, and the epitaph below the date reads "Born in fire, died in fire", also in Russian.
To prepare for his role, Ron Perlman read all of the Hellboy comics, and worked out three hours a day, five to seven days a week (varied). He also worked out while shooting, every day he had off from filming, he would workout.
The scenes, where Abe is in the medical tank, were shot on Doug Jones' birthday. He spent several hours in a harness to make it look like he was floating in water. By the time he was taken out of the harness, he was all scraped up and bloody from it. That morning, Guillermo del Toro greeted Jones with "Happy Birthday, Doug. Today, we are going to hang you up by your balls." as told by Guillermo del Toro in the DVD featurette.
This movie takes its story from more than one Hellboy comic. The meat of the story comes from the "Seed of Destruction" storyline, but the rest is taken from the "Right Hand of Doom" and "Box Full of Evil" short stories. The movie also contains little homages to other stories such as "The Corpse" and "Pancakes".
When Make-up Artist Matt Rose at Cinovation studios wanted to start working on the make-up design for the Hellboy character, they went into their archives and found that they already had a life mask of Ron Perlman from when Rick Baker had made his prosthetics for Beauty and the Beast (1987). Some slight changes were required to account for Perlman having aged sixteen years.
When Professor Bruttenholm (Sir John Hurt) is showing Agent Myers (Rupert Evans) through the B.P.R.D. when he first arrives, a greyish male humanoid statue with a large ring on its groin is momentarily seen. This is Roger the Homunculus, a supporting character introduced late in the Hellboy comics (in the story arc "Almost Colossus"), who often goes through periods of dormancy. When active, he is a Special B.P.R.D. Agent, much like Abe. Roger did not make it into the movie script as a character, but made it into the film as a piece of set decoration instead. Another comic book prop reference in the same scene is the large set of metal boots sitting in one of the glass display cases. These belonged to a supernatural creature called "The Iron Shoes" from the short story of the same name.
The scene in the abbey at the beginning of the movie was actually shot in Prague, not Scotland. It was one of the coldest Aprils on record. It was so cold, that icicles formed on the branches of the trees, causing some of the trees to fall.
The Spear of Longinus seen briefly in the movie is an exact replica of what is thought to be the actual Spear (which is in the possession of the Hofsburg Treasure House in Vienna), right down to the golden sheath wrapped around its center.
Ron Perlman wore "booties" made of greenscreen material, for the scene where we finally meet Hellboy as an adult, in which Hellboy's cloven hooves were revealed, but due to the low-key lighting, his hooves are a little hard to see.
According to the deleted scenes on disc two of the two-disc Special Edition DVD, the obelisk that serves as the lock by which Hellboy would release the Ogdru Jahad, fell to Earth during the "Tunguska event", an enormously powerful explosion, believed to have been caused by the air burst of a large meteoroid or comet, that occurred near the Podkamennaya (Lower Stony) Tunguska River, in what is now Krasnoyarsk Krai, Russia, on June 30, 1908.
The U.S. soldiers' unit patches identify them as from the 2nd Infantry Division, the "Indian Head" division. In real-life, they were defending the town of St. Vith, Belgium, during the events in the opening scene.
EASTER EGG: When Agent Myers first meets Abe Sapien, Abe asks Myers to turn the page. If you move your mouse over to the bottom right hand corner where Myers is, you will see a Hellboy comic appear and when you click on it, it will give you a short biography on Abe.
When Agent John Myers first gets to the front gate of the B.P.R.D., he has to get past a security check. The voice of the security guard is Property Master Michael Lindsay, who was standing behind the pillar during the shot.
The biggest challenge for the location scouting, was finding an area to film the New Jersey rooftop scene, as it was proven that Prague didn't have many locations that can serve as an American city. After many false starts, Del Toro finally found the right street for the scene to be filmed, which had to be redressed to look more American.
In Hellboy's room, there's a Ghost poster, another Dark Horse character that deals with the supernatural. The character Ghost has teamed up with Hellboy in a two issue crossover comic that was published from May to June 1996.
The motto, depicted in Latin in the lobby floor, of the Bureau for Paranormal Research and Defense reads: "In Absentia Luci Tenebrae Vicunt". Grammatically correct it should say: "In Absentia Lucis Tenebrae Vincunt" which loosley teanslates to: "In the absence of light, darkness prevails."
EASTER EGG: On disc two, go to the Board A Matics screen. Put cursor on back, then press down. You will see what looks like three drops appear. Press "Enter" to see a wireframe animation of the apocalypse scene.
Trevor Broom is revealed early in the movie to be gravely ill from cancer (in the Director's Cut DVD, he is revealed to have only a few months left, and it is a malignant sarcoma in his lungs and spine). Sir John Hurt, the actor who played him, died from pancreatic cancer.
For years, fans of Ron Perlman have noticed similarities between Ron and experimental rock/jazz/industrial musician Tom Waits. When Agent Myers first enters the vault to meet Hellboy for the first time, Tom Waits 'Heartattack and Vine' song (from the album of the same name) can be heard playing. Perhaps a joke by the actor and director, regarding the similarities in appearances. Tom would later film a music video for 'Goin' Out West' from his Bone Machine album where his resemblance to Ron has been mentioned over the years.
The trivia item below may give away important plot points.
The idea of Rasputin getting stabbed in the belly, with a creature coming out of the open wound, was an abandoned idea for the Hellboy comic book story Seed of Destruction. Mike Mignola never mentioned this to del Toro, and was surprised that del Toro came up with the same idea for the movie.