1. Sarah Michelle Gellar was up for the lead in Sabrina, the Teenage Witch before accepting the role of Buffy.
2. Although she almost wasn't even Buffy. She was originally up for the role of Cordelia, while the actress who played Cordelia, Charisma Carpenter, auditioned for Buffy. Producers made them swap roles.
3. Carpenter was reluctant to take the role of Cordelia as she had already played a bitchy character in Aaron Spelling's Malibu Shores. Her agent told her she couldn't be typecast because no one knew who she was, so she admitted defeat and signed up.
4. Carpenter was 27 years old when the show first aired, making her one of the oldest actors ever to play a teenager.
5. Selma Blair and Katie Holmes were also up for the role of Buffy.
6. Ryan Reynolds almost played nerdy sidekick Xander – but ended up turning it down.
7. Anthony Head, who played Rupert Giles, was the first actor cast in the series. Producers knew on the first day of casting he was right for the role.
8. The role of Willow was recast after the pilot. Producers didn't feel the original actress was quite right so enlisted Alyson Hannigan once the show got picked up.
9. The show was actually a sequel of sorts to a movie of the same name. Showrunner Joss Whedon wrote the script for the film, which was produced in 1992. A few years later he was approached about turning it into a TV series and the show was born.
10. Donald Sutherland, Luke Perry, Hilary Swank, Seth Green, Ricki Lake, and even Ben Affleck (as a bit-part basketball player) all appeared in the 1992 movie.
11. David Boreanaz was discovered by the casting director's friend. They were struggling to cast the part of Angel until a friend spotted Boreanaz walking his dog and got him to audition because he fit the bill so much.
12. And it's a good job they did, as he provided many laughs on set. Boreanaz used to pull his pants down between scenes to test if his co-stars could keep a straight face and stay in character.
13. He also used to fool around with Gellar, his on-screen love interest. In a 2002 interview, Gellar revealed: "[We] were the worst. We would do horrible things to each other. Like eat tuna fish and pickle before we kissed. If he had to unbutton my shirt or trousers I would pin them or sew them together to make it as hard as I could. Once I even dropped ice cream on him."
14. Boreanaz was reportedly offered the role of Batman in Christopher Nolan's reboot as a result of his portrayal of Angel, as well as a role in the Resident Evil series – but allegedly turned both down.
15. Nicholas Brendon, who played Xander, grew up with a stuttering problem and got into acting to help overcome it.
16. As the show went on, Brendon was told to stop working out as he was getting too fit to portray Xander's nerdiness.
17. The character of Jenny (Robia LaMorte) was originally called Nicki, but it became too confusing as everyone on set called Brendon "Nicky", so they switched her name to Jenny.
18. LaMorte started off as a dancer, and appeared in Prince's music video for 1991 single "Cream".
19. In the show, Willow says she used to write fan-fiction about TV show Doogie Howser, M.D.. Hannigan ended up acting opposite Doogie Howser himself, Neil Patrick Harris, years later in How I Met Your Mother.
20. James Marsters, who played Spike, wasn't actually British. He's from California and auditioned with a Texas accent, but producers decided it would be better if the character was a Londoner. Head served as his dialect coach throughout the series.
21. Gellar sometimes struggled with the dialogue, which was branded Buffyspeak and inspired by California Valley girls.
22. In fact, there is a book out explaining the slang in the show – Slayer Slang: A Buffy the Vampire Slayer Lexicon.
23. Whedon knew he wanted either Willow or Xander to come out as gay, but it took him a while to eventually choose Willow.
24. Darla, played by Julie Benz, was supposed to die in the second episode. But Whedon kept her alive to make the Buffy/Angel romance a bit more interesting.
25. The show was shot in a warehouse and actual schools because they had such a tight budget in the early days.
26. But producers later built their own graveyard in the warehouse's parking lot to make their life easier for shooting scenes.
27. The cast hated filming all of the scenes in the library because it would always mean a lengthy speech for Head.
28. Dolly Parton was (technically) an executive producer of the show. One of the executive producers, Sandy Gallin, had a longstanding production partnership with the iconic country singer.
29. Buffy the Vampire Slayer was never up for an Emmy award in any of the major categories. Well, apart from a token writing nod in 2000.
30. The original theme song was scrapped as Whedon was unhappy with it. So he opened up a contest for local indie bands to come up with ideas. The final choice was written and performed by Nerf Herder, who had been recommended by Hannigan.
31. A few actors played more than one villain. For example, actor Camden Toy played one of the Gentlemen in "Hush" in Season 4, and in Season 7 he played both a demon called Gnarl (in the episode "Same Time, Same Place") and Ubervamp Turok-Han.
32. There have been academic courses on the series, as well as books and conferences dedicated to the themes of the show.
33. There was a reason for the vampires' scary faces – Whedon said: "I didn't think I really wanted to put a show on the air about a high-school girl who was stabbing normal-looking people in the heart. I thought somehow that might send the wrong message, but when they are clearly monsters, it takes it to a level of fantasy that is safer."
34. Whedon had a special nickname for Gellar due to how much pain she had to deal with on screen. He said: "We used to call her Jimmy Stewart, because he was greatest American in pain in the history of film."
35. Originally, viewers were never supposed to see Buffy's family, but it was soon realised that this wouldn't be convincing enough.
36. Buffy's younger sister, Dawn, was originally meant to have special powers of her own. Producers had planned for her to be able to communicate with the dead, but this was decided against before filming commenced.
37. Whedon claimed Buffy's mother was the hardest character for him to kill off.
38. The Buffy stories continue in the form of comic books. Some of the TV writers also work on the comics.
39. The show almost became an animated series, but no one wanted to pick it up.
40. While she was on the show, Gellar purposefully tried to keep out of the public eye so young fans wouldn't see Buffy with a drink or cigarette in her hand.
41. Eliza Dushku (Faith) grew up a Mormon and therefore refused to ever do any nude scenes in the show.
42. The rest of the cast only found out Buffy the Vampire Slayer was ending when Gellar announced it in the media.
43. Britney Spears almost guest-starred. She was lined up to play robot April in Season 5, but her schedule wouldn't allow it.
44. Oz, played by Seth Green, was the only character based on someone Joss Whedon knew.
45. Buffy got turned into a rat in Season 2, partly because Gellar needed time off to appear on her first Saturday Night Live slot.
46. Gellar's future husband, Freddie Prinze Jr, wanted a role in the show, and almost played Dracula in Season 5.
47. At one point, before Season 7, Gellar and Brendon proposed a romance storyline between Buffy and Xander to Whedon. But he wanted to keep them as platonic friends.
48. Sarah Michelle and Alyson are the only two Buffy stars who appeared in every episode of the show (144 in total). Nicholas Brendon missed one episode.
This post has been updated to meet BuzzFeed's sourcing and attribution standards. One item in the list, 14, has been updated to reflect the fact that it is a rumour, not a fact.