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.
Actor Allusion: Giles piping in "Dance Macabre" during his slideshow presentation in "Hush". Anthony Stewart Head appeared in the pilot of Jonathan Creek, which used this song as its theme.
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."
In "Nightmares", Buffy's personal nightmare is being buried alive by the Master. Sarah Michelle Gellar is terrified of graveyards for this same reason, which is amusing seeing as her character was buried alive twice in the show ("Nightmares" and "Bargaining Pt. 1"), and spends most of her evenings in graveyards.
Baby Buffy went through a phase during which she idolized ice skater Dorothy Hamill. Sarah Michelle 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.
Sarah Michelle Gellar disliked Season 6 because of the dark tone of the storylines. She also wasn't a fan of the extensive preparation that had to be done for the Musical Episode.
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.
Michelle Trachtenberg was a huge fan of Buffy before being signed onto the show, and once told Joss "There was never a Buffy episode that sucked. I love you!"
When "Lover's Walk episode originally aired in the US, it was followed by an advert for the phone service 1800-COLLECT, in which Sarah Michelle Gellar and David Boreanaz offered a prize of a walk-on part in "The Prom."
Real-Life Relative: Nick Brendon's twin brother worked as his stunt double, and they also brought him in to play Xander's magically created double in season five's "The Replacement".
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.
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).