From the Series as a whole:
- Recursive Adaptation: Hellboy Animated has its own comic books.
- Write Who You Know: Hellboy himself is largely based on Mike Mignola's father, who was a cabinet maker. He'd often come home to his family with tales of horrific on-the-job accidents, including one careless fellow losing a hand, told in the nonchalant, unflappable manner that would become HB's trademark.
From the Comics:
- Author Existence Failure: Industry legend John Severin was asked what types of story he'd like to draw when he came onboard. His first request, to do a Western, led to the Witchfinder story "Lost and Gone Forever". His second request, to do a WWII story, led to the Sledgehammer '44 series being developed, but he died after only drawing the first three pages and some character designs. The series was eventually completed by other artists.
- Creator's Pest: Both Mike Mignola and Duncan Fegredo realized they've made a huge mistake after they designed and debuted Nimue's three raven helmet and learned how awful it was to draw it. This led to not only Nimue wearing said helmet making few appearances in ''The Storm And The Fury," but Mignola also wrote in a scene where the helmet got ditched so Fegredo no longer had to illustrate it.
- Exiled from Continuity: The appearance of the Torch of Liberty in the first miniseries is still canon, but since the character is created and owned by John Byrne, who scripted the original miniseries, he has never reappeared outside of Hellboy's "family photo".
- Flip-Flop of God / Lying Creator: In the miniseries Hellboy in Hell, Mignola released a heartfelt letter saying this was going to be the end of Hellboy, and that he wrote issue ten as such. A few months afterwards, an issue of BPRD showed Hellboy was coming back and Mignola released a statement contradicting his earlier statement by saying Hellboy in Hell was never planned to be the end and that Hellboy will indeed return for the apocalypse promised in King of Fear. Either Mignola changed his mind or he was lying about Hellboy in Hell being the end.
- Writing by the Seat of Your Pants: Mignola plots the series years in advance, but he did change his plans for Hellboy In Hell which was originally intended to run at its own pace indefinitely. But after Hellboy killed Satan too many dominos were pushed over and he decided to go for a Grand Finale with issue ten instead.
- What Could Have Been: Mignola originally intended to Liz to die after her encounter with Roger, due to lack of ideas at the time on where to take the character.
From the films: