Acting for Two: In the flashback, the medieval Slayer and her Watcher are played by the same actors as Buffy and Merrick. On the other hand, the Slayer and the Watcher continually reincarnate looking identical to her past lives, so they are theoretically still the same characters.
Fake Nationality: The medieval maiden played by the California-born Swanson in the flashback sequences would obviously have to be non-American. She comes off as English(not that Swanson tries very hard), but a brief line by Merrick suggests that she was Hungarian. (In the original script, she's an Italian.)
Producers were reluctant to cast Amber Benson as Tara because she was too voluptuous for the supposedly plain girl, but she won them over with her awkwardness.
Alyson Hannigan's casting went something like this too. Joss stated that all the other girls who auditioned for Willow wore Nerd Glasses and acted stereotypically nerdy. Alyson won the part based on the fact that she didn't do that.
James Marsters based his accent on Anthony Stewart Head's. Giles uses an RP English accent on-camera, but his actor sounds like a working-class "tough guy" in real life, using an accent close to Estuary English. Head uses it for Giles' adolescent persona in "Band Candy."
Baby Buffy went through a phase during which she idolized ice skater Dorothy Hamill. Gellar enjoys figure skating in real life, a skill which was put to good use in "What's My Line Pt 1".
The scene in "Lovers Walk" where Cordelia gets impaled is based on a real incident. At five years old, playing around a swimming pool that was still under construction, Charisma Carpenter fell onto a piece of rebar. Only in real life she fell off a horse and not through stairs.
Faith became a Boston native after Eliza Dushku was cast, though Eliza herself is actually from Watertown rather than South Boston like Faith.
Most of the cast came to hate the library scenes because they would take forever to film.
James Marsters was so disturbed by Spike's attempted rape of Buffy that he had it written into his contract that he would never film such scenes again.
Seth Green disliked being Out of Focus in Season 3, as he hated having to show up for work when he'd only have one or two lines the whole time. This is why Oz gets Put on a Bus midway through Season 4.
Creator's Favorite: Xander (who was a stand in for Joss), Dawn (with her actress being such a big fan of the series, focus was given to her), and Kennedy (Whedon really liked the character and gave a lot of focus to her) being standouts.
The Danza: Larry Bagby III as Larry Blaisdell. DB Woodside as Robin Wood.
The Other Darrin: Nikki Wood, the second slayer Spike kills, is played by a stuntwoman in "Fool For Love." When The First assumes her shape in Season 7, she was recast as a more vulnerable-looking actress.
Buffy's old mentor Merrick looks different in a season 2 flashback.
The Other Marty: Riff Regan played Willow in the unaired Proof of Concept to sell the show but never intended to play her in the series.
The famous Mutant Enemy title card was reportedly scribbled by Whedon with only seconds to go until broadcast.
In "Innocence", The scene where Angel dumps Buffy in his apartment was originally supposed to take place on Buffy's lawn. Halfway through shooting the scene, SMG and David Boreanaz told Joss that the scene wasn't working, so he moved it to the bedroom. Buffy can still heard saying she saw him "at the house". She was so Lost in Character afterwards they had to close down the set for half an hour, being that invested in the romance.
The gag in "Earshot" with Giles walking into a tree was ad-libbed by Anthony Stewart Head. He never dreamed Joss would keep in the cut.
In the Season Three finale, Cordelia cowers behind the diminutive Jonathan. According to Danny Strong, this scene was an idea of Charisma's, because she was leaving for Angel and never got much screen time with him.
Too Soon: The episode "Earshot", which depicts a student bringing a rifle to school (albeit to kill himself and no one else), was set to air within a week of Columbine and was therefore delayed until after the season finale, which was also delayed in light of Columbine because of the destruction of Sunnydale High School.
Throw It In: Not in the original work but in the motion comic Anywhere But Here there is a scene where Kennedy guesses Willow is upset about Tara's death. A line's added that makes her look like Queen Bitch.
At the height of its popularity, a animated version was considered. They had got the designs down and even made a promo. But no network was willing to buy it and it wound up dropped in the end. Though it did get a nod in the season 8 comic which somewhat lampshades the missed opportunity.
Joss Whedon planned to make a spin-off called "Ripper", about Giles' adventures in England. At first it was meant to be a show, then a mini-series, then finally a made-for-TV movie. Apparently, he still hopes to make it.
After the series ended, Eliza Dushku was offered her own spin-off series as the character Faith. But then she turned it down to do Tru Calling.
Ryan Reynolds was originally offered the role of Xander. He passed on it because of his own awful high school experiences. "I love that show and I loved Joss Whedon," Reynolds told 'The Toronto Star' in 2008. "But my biggest concern was that I didn't want to play a guy in high school."
Nathan Fillon auditioned for Angel. He played Caleb in the last season.
Dawn was originally intended to have the power to speak to the dead. She was also supposed to be able to move objects with her mind. These powers were later dropped.
Originally, Whedon didn't want either of Buffy's parents to appear as characters on the show. Accepting that that could get complicated, he settled on just having her mother, Joyce, appear in a nearly regular role, while Buffy's father Hank appeared in very few episodes.
Oz was originally supposed to wear thick rimmed glasses, much like the lead singer of Weezer. This was dropped at the last minute as Whedon felt that it looked tacky.
Whedon toyed with the idea of making Xander gay instead of Willow.
Spike, Oz, Faith, Wesley and Drusilla were all supposed to be killed off, but have ended up living long past their initial storylines. Also, Jenny and Joyce were supposed to be killed off sooner than they were.
The Oz-Veruca storyline in season four was supposed to last longer, but had to be cut short when Seth Green left the series.
According to Nicholas Brendon, he and Sarah Michelle Gellar proposed a storyline for season seven where Xander and Buffy get together romantically. Their idea was shot down, and they continued with the Buffy-Spike relationship.
The Master was originally supposed to have a beard and long hair before Mark Metcalf suggested the bald look as a homage to Nosferatu.
James Marsters originally auditioned for Spike with a Texan accent. This was quickly jettisoned in favor of a London accent instead.
You Look Familiar: Spice Williams plays the police instructor who tries to murder Buffy, Patrice, in "What's My Line?" She also appeared in "The Zeppo" as a member of the Sisterhood of Jhe, and later turned up on Angel as a convict hired to kill Faith in prison.
Jeff Kober first appeared in season three episode "Helpless" as a vampire named Kralik. He appeared again during season six as warlock Rack.
Brian Thompson portrays the vampire "Luke" in season 1, aswell as the demon called "The Judge" in season 2.
Tom Lenk plays a vampire lackey to Harmony a few season before playing Tucker's brother (and a member of the Trio).