The film version pulls heavily from the "Seed of Destruction" mini-series to provide audiences with Hellboy's origins. Del Toro chose to elaborate on some of the themes that were not really developed in the comic.
The film expands on the father-son relationship between Professor Broom and Hellboy. The film also introduces a love triangle between Hellboy, his fellow B.P.R.D. agent Liz Sherman and the young FBI Agent John Myers. Del Toro created the character of John Myers to facilitate this plot point.
These choices are often difficult for a fan like del Toro, who is so passionate about all the nuances of the story. But whenever he ran into difficulty, del Toro had the ultimate guide in Mike Mignola.