Martin Flagg is a movie writer who rewrote Hell Hazers II: The Reckoning.
During the filming of one scene Marty, sits next to McG when Brad asks McG, how the ghosts hear the chanting if they are in hell. McG told Marty that he is the writer, Marty said, "what if I throw in an explainer?" Brad said that would be great, and he leaves, focused on his Bluetooth. Marty was one of the witnesses of Brad's death.
During further filming Marty sits next to Jay, an executive producer. Marty asks Jay if we shouldn't shut it down after Brad's death. Jay said that they had a moment of silence for him at breakfast and that he was just a studio guy.
During a cut Tara talks with McG why the ghosts are afraid of salt. McG then asked Marty "what else would a ghost be scared of?" Marty said may be shotguns, and Walter Dixon who was standing next to Dean, angry said that these people are idiots and left.
After Jay's death Sam and Dean went to the office of Martin Flagg to talk to him about the summoning rituals and authentic language in his script. Marty said that he uses the original script for "Lord of the Dead" from Walter Dixon but Marty changed ninety percent of it to make it readable.
Dixon lures Flagg onto the set to kill him, but Sam and Dean arrive in time to save him. Walter admits that he was conjuring real ghosts and forcing them to kill those he saw as responsible for ruining his script. Before they can stop him, Walter destroys the talisman he was using. This frees the spirits who, enraged at being used, turn on Walter and kill him. The production of the movie continues, with Martin incorporating his experiences with the ghosts into the script.