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.
Upon meeting to discuss the movie, Hellboy creator Mike Mignola 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.
The scenes where Abe is in the medical tank were shot on Doug Jones's 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, Director 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.
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, actor 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.
Very early on in the film's preproduction, 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.
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.
To prepare for his role, Ron Perlman read all 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.
When Professor Bruttenholm is showing Agent Myers through the BPRD when he first arrives, a greyish male humanoid statue with a large ring on its groin is momentarily seen. This is actually 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 BPRD 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.
Director 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 Hellboy 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".
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.
It took anywhere from five to seven hours to apply Doug Jones's 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.
When the 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 16 years.
The US soldiers' unit patches identify them as from the 2nd Infantry Division, the "Indian Head" division. In real life, they were actually defending the town of St Vith, Belgium during the events in the opening scene.
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.
Ron Perlman wore "booties" made of green screen 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.
The biggest challenge for the location scouting was finding an area to film the New Jersey rooftop scene. As it was proved that Prague didn't had 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.
According to the deleted scenes on disk two of the two-disk 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 in Russia on June 30, 1908.
When John Myers first gets to the front gate of the BPRD, he has to get past a security check. The voice of the security guard is the property master, Michael Lindsay who was standing behind the pillar during the shot.
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.
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.
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 the lobby floor, of the Bureau for Paranormal Research and Defence (Defense) is: "In Absentia Luce Tenebrae Vincit". Meaning, in the absence of light, darkness prevails (or triumphs).
EASTER EGG: On Disc 2 go to the Board A Matics Screen. Put cursor on back, then press down. You will see what look like 3 drops appear. Press Enter to see a Wireframe animation of the Apocalypse scene.
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.