Walter Dixon was the original writer of the Hell Hazers II movie, but it was changed by the studio. His script was originally called Lord of the Dead. Because of his contract, he was allowed on set. At some point in his life he studied the dark arts, and wrote that into his script.
Walter was the original writer, but because it was changed so much, it was not his story anymore. He was seen briefly around set, and Sam and Dean noticed him, but they thought he was a PA. They found out that his script was like a "How To Summon spirit" manual. He used his knowledge to summon ghosts and used them to kill those who interfered with his masterpiece.
Back on set, Walter took out his anger on the guy who rewrote the script, and summoned spirits to kill for him. Sam and Dean tried to stop him. Sam then chased after Walter. He tried to stop Walter, but he was too late. Walter smashed the Talisman he had used to control the ghosts without realizing what he had done. The ghosts got their revenge on Walter as they shredded him to death.