Here's how a comic miniseries helped tie the events of the Buffy The Vampire Slayer movie into the TV show. Buffy Summers had a bumpy ride before she became a TV icon. The franchise started life with a 1992 movie, which cast Kristy Swanson in the title role, with Rutger Hauer and Paul Reubens vampire villains giving her a hard time. The screenplay was written by Joss Whedon, but unhappy over numerous rewrites and tonal changes to make it lighter and campier, he walked off the project.
The movie did lukewarm business but Whedon got a chance to revive the concept with the TV show, which arrived in 1997. Buffy The Vampire Slayer cast Sarah Michelle Gellar in the role, with the show combining horror, comedy and high-school drama with a rich mythology and likable supporting characters. The series was hugely popular and spawned spinoff Angel, comics and video games. The series came to an end in 2003 after seven seasons, and a reboot is currently in development.
The Buffy The Vampire Slayer 1992 movie is now something of a curio for fans to seek out, but it's not considered canon as the show contradicts it in several key ways. Parts of Buffy's movie backstory were later included on the show, with season 2's two-parter "Becoming" featuring flashbacks of her first Watcher Merrick and her life in Los Angeles before moving to Sunnydale. In 1999 a three-part comic series called "The Origin" took Joss Whedon's original screenplay and merged Buffy's origin with the TV series.
Buffy The Vampire Slayer's "The Origin" is written by Dan Brereton & Christopher Golden, and drops the goofy elements of the movie in favor of the show's darker tone. Major plot elements that were dropped from the film to Whedon's chagrin, such as Merrick killing himself rather than letting villain Lothos use him against Buffy, were restored. "The Origin" is far from perfect, however, with the story feeling rushed and still presenting inconsistencies with the series.
It did, however, do a solid job of recontextualizing the Buffy The Vampire Slayer movie as an actual prequel. Joss Whedon is relatively strict when it comes to assigning canon to offshoots, but he did confirm in 1999 that "The origin comic, though I have issues with it, CAN pretty much be accepted as canonical. They did a cool job of combining the movie script (the SCRIPT) with the series, that was nice, and using the series Merrick and not a certain OTHER thespian who shall remain hated." The latter jab at Donald Sutherland's expense comes from Whedon's bad experience with the veteran star, who played Merrick and was reportedly rude to the cast and crew and would rewrite his own dialogue on a whim. "The Origin" comic made Merrick closer in line to the Richard Riehle version of the character seen on the show.