Doug Jones' (Abe Sapien's) voice was dubbed by David Hyde Pierce, but Pierce refused a credit because he felt that Abe was entirely Doug's creation, and did not wish to detract from his performance.
When writer and director 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, similar to the Hulk. Another suggestion was that he came from Hell, but was a normal human. Del Toro vetoed all such attempts to alter the character.
Much of the demonology in this movie was 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.
Upon meeting to discuss the movie, Hellboy creator and co-executive producer Mike Mignola and writer/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.
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.
Doug Jones: out of make-up, as one of the patients at Bellamie Psychiatric Hospital, aside from his Abe Sapien role.
Mike Mignola: Hellboy's creator appears dressed as a knight in a crowd of costumed people menaced by Sammael.