"In developing the Doom script, Callaham said that the games themselves helped in terms of the visuals that he wanted to put in. However, he said, "In terms of character development it is non-existent (in the games) and the id guys are the first to admit that." Callaham said that in his meetings with the id Software team they were very supportive and open with his ideas on changing the Doom mythology and allowing for real characters to be put into the script. "They were not clinging to anything." he said, "They were really quite generous in changing the Doom mythology a bit." One thing that id did insist upon is that the main character in the movie be named John. When asked, Callaham was unsure if that was a tribute to id’s master programmer John Carmack but did admit there is another character named Carmack in the movie.

And what about those newly created characters? Callaham said that John, the Space Marine to be played by Karl Urban (best know as Eomer in the Lord of the Rings trilogy), is stationed along with his unit in a research base on an alien planet (the planet is unnamed in the movie unlike the games Doom and Doom 3 which are set on Mars). In the movie he is reunited with his sister, a scientist on the based named Samantha (to be played by Die Another Day villainess Rosamund Pike). They were separated after an accident that killed their parents and Callaham says, "They are a little estranged" However, strange things are afoot on the base as alien monsters begin to appear and both brother and sister have to put aside their differences in order to survive. Dwayne "The Rock" Johnson will also appear as Sarge, the head space marine, but Callaham warns, "He is not going to be smiling a whole bunch. You won’t see him raise his left eyebrow."