“Supernatural” was initially supposed to be an anthology about American urban legends like "The Twilight Zone" or "American Horror Story.” However, creator Eric Kripke decided to change course, first to a story about two journalists, and then to two brothers who hunt demons and ghosts.
The birthday of the main character, Sam Winchester, is October 2, 1983, and in a spooky coincidence, Kripke’s son was also born on October 2, 2007. Sam's long-dead girlfriend, Jessica, and Kripke’s wife share a birthday on January 24, and his own birthday is the day the Impala "rolled off the lot.”
Jared Padalecki and Jensen Ackles did their first read together and were the only two chosen to audition in front of network executives for the parts of the Winchester brothers. It didn't take more than a chemistry read for the two of them to get the parts, and they stuck with the roles for 15 seasons.
Misha Collins initially thought he was auditioning for the role of a demon, but he adjusted his performance when Kripke revealed that he'd actually be playing an angel. His character, Castiel, was eventually possessed by the Devil, and Collins portrayed the angel for just over a decade.
Unsurprisingly, "Supernatural" went through multiple showrunners during its 15-seasons on the CW. Each showrunner made the show feel different than before, adding new characters and further expanding the mythology as they went.