Originally, the brothers were raised by their aunt and uncle. Thus, when Dean comes to Sam for assistance in the pilot episode, he has to convince him that the supernatural exists. However, Kripke realized that this made the backstory too complicated, and reworked it with Fox executive Peter Johnson so that their father raised them to be hunters. The script went through many additional revisions. One of the original ideas was for Sam's girlfriend Jessica being revealed as a demon, which prompts him to join Dean on the road; however, Kripke felt it was more appropriate for Sam's motivation to be Jessica's death, so he had her killed in the same manner as Sam's mother. Other revised concepts include Sam believing Dean to be a serial killer that murders their father, and their father dying in Jessica's place. Filming for the pilot episode was greenlit after director David Nutter, who previously had worked with Kripke on Tarzan, signed on. Wonderland owner McG signed as executive producer. When the series was eventually picked up, the studio brought on Robert Singer as executive producer, as it wanted Kripke to work with someone with production experience. Co-executive producer John Shiban was also hired to help design the series mythology due to his previous work on The X-Files.
Kripke originally intended for the car to be a '65 Mustang, but his neighbor convinced him to change it to a '67 Impala, since "you can put a body in the trunk" and because "You want a car that, when people stop next to it at the lights, they lock their doors."
Kripke has commented, "It’s a Rottweiler of a car, and I think it adds authenticity for fans of automobiles because of that, because it’s not a pretty ride. It’s an aggressive, muscular car, and I think that’s what people respond to, and why it fits so well into the tone of our show."
Kripke had the series planned out for three seasons, but later expanded it to five, and hopes to end it there on a high note. Now the network has announced a sixth season, but confirmed that Kripke won't be present in the production.
