The studio wanted Vin Diesel to play Hellboy.
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.
To try and convince the studios of Ron Perlman's star quality, director Guillermo del Toro showed the studio executives footage of Perlman's character (Reinhardt) in Blade II (2002).
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.
Baby Hellboy, Sammael, Ivan the corpse, Train Driver and Kroenen were all voiced by Guillermo del Toro.
The scooter that Myers is seen riding around on was given to Rupert Evans as a gift from producer Patrick J. Palmer.
When Prof. Broom holds the two ripped pieces of paper he found on Kroenen, they initially form the shape of a yellow Star of David - which the Nazis required Jews to wear for identification.
It was Del Toro's idea for Kroenen to have a mechanical hand.
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.
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.
None of the cast member's names are written on the posters, mentioned in the trailers or shown in the opening credits for this movie.
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.
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.
Ron Perlman broke a rib while filming the subway scenes. He jumped onto a train that was coming towards him at about 45 mph.
In addition to being considered for the role of Hellboy, Vin Diesel was also considered for the part of Abe Sapien.