- Acting For Three: Doug Jones plays Abe Sapien, The Angel of Death and Chamberlain.
- Actor-Shared Background: Luke Goss (Prince Nuada) has a twin in real life, much like the character he plays in this movie.
- Cameo Prop: The damaged mask of Kroenen from the first film can be seen in a glass case at the B.P.R.D. Headquarters.
- Channel Hop: A rare film example; switched from Sony Pictures to Universal. This was due to Revolution, who produced the first film and had an exclusive deal with Sony, had folded at the time.
- The Other Darrin: Abe is now voiced by his actor, Doug Jones, rather than David Hyde Pierce. Pierce felt Jones' performance was strong enough that there was simply no need to hire a separate voice actor. The difference was practically unnoticeable, since Pierce had only been cast in the first place because some executives thought Jones wasn't a big enough "name", and he'd been mimicking Jones' performance anyway.
- Real-Life Relative: Blake Perlman (Ron's daughter) can be seen in the sequel as the reporter in the gray sweater interviewing Hellboy outside the auction house.
- Scully Box: Brian Steele, who portrayed Wink, wore ten-inch stilts (going from his towering 6'5" frame to 7'3") while in the Wink suit.
- What Could Have Been:
- Charlie Hunnam was actually del Toro's first choice to play Nuada. However, Nuada's design looked too extreme on him so Luke Goss was cast instead. Hunnam would later work with del Toro in Pacific Rim.
- Christopher Lee was originally offered the role of King Balor.
- The film originally ended with a Sequel Hook involving Roderick Zinco (from the comics) obtaining Kroenen's severed head, traveling to a secret Nazi safehouse in the Arctic, attaching it to a massive robot body, and Rasputin's ghost manifesting before them. It was included as a motion comic on the Golden Army DVD.
- The role of Manning was originally intended for Larry Miller when Jeffrey Tambor was unavailable.
- One idea del Toro had was Hellboy facing the classic versions of Frankenstein, Dracula and The Wolf Man.
- Hellboy III, which has sadly been confirmed dead by del Toro in a February 2017 interview after a long time in Development Hell.
Trivia / Hellboy II: The Golden Army