Let's get this out of the way up front: the 1992 movie on which Buffy is based isn't great. If it weren't for the TV show that followed, the Kristy Swanson-starring flop would have been quickly and quietly forgotten, fading away into the background of D-rated horror movies that you find after a few too many hours in a Netflix spiral.
So how did the story get a second act? Flash back to 1989, when young Joss Whedon is working as a staff writer on Roseanne, where he is, by his own admission, bored. He's a staff writer, the lowest position in the writer's room, and he only gets to write a few scripts a season, leading him to pursue an outside project: Buffy. The idea came from a picture in Whedon's head of the pretty blonde girl in a horror movie who always walks into an alley and gets killed. "I felt bad for her, but she was always more interesting to me than the other girls," he told Rolling Stone. "She was fun, she had sex, she was vivacious. But then she would get punished for it. Literally, I just had that image, that scene, in my mind, like the trailer for a movie— what if the girl goes into the dark alley. And the monster follows her. And she destroys him."
While Swanson was an apt enough Buffy, the movie was hampered by some of its supporting cast, particularly Donald Sutherland, who played the Slayer's original watcher, Merrick. According to Whedon, Sutherland was awful to work with on set, frequently changing his lines and just generally having "a very bad attitude" on set. That, coupled with the fact that director Fran Rubel Kuzui had a different vision for the film, made the project uncomfortable for Whedon to work on.
"I've never had a worse experience in my life, and I've often thought of doing a lecture series on how to make movies based on just showing that movie, because I think they literally did every single thing wrong," he told The AV Club in 2001 of the experience. "The production design, the casting, there wasn't a mistake they left unturned… I literally didn't see any of it again until I saw the director's cut, during which I actually cried… I said, 'I can't believe this.' I was heartbroken."
The idea to make the movie into a TV show came from Gail Berman, who owned the rights to the movie and approached Whedon about turning it into a show instead. Realizing they had similar ideas, the two crafted a pitch they presented to FOX and NBC. Both networks rejected the show, but it ended up finding a home on the upstart network the WB, who ordered a presentation from the pair, but not a pilot. They eventually ordered the show as a midseason replacement—and the rest, as they say as history.
The Buffy movie didn't fare well in theaters and it isn't considered canon by fans of the show, but it did help to launch the careers of quite a few famous actors. Academy Award winner Hilary Swank counts Buffy as her first film credit, and the movie also offered early parts for David Arquette, Thomas Jane, Luke Perry, and Ben Affleck. Seth Green, who would go on to appear on the Buffy series as fan favorite Oz, also had a small role as a vampire.