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.
Deleted Role: Seth Green filmed some scenes as a vampire but most of them were cut. He can be seen from behind and briefly when Buffy kicks him outside the gym.
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.)
Amilyn was written to be female and played by Joan Chen. When she dropped out, the character became male instead.
In the original version of the script, Merrick committed suicide to escape being turned into a vampire by Lothos, who wanted to know the Slayer's identity so he could dispatch with her. It is this version that was restored in the Buffy "Origin" comic.
Producers were reluctant to cast Amber Benson as Tara because she was too voluptuous - they wanted someone daintier and waifier to take Willow's place as The Woobie. But Amber won them over with her ability to play awkward and vulnerable.
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 (including Riff Regan, who played her in the pilot). Alyson won the part based on the fact that she didn't do that.
Acting in the Dark: Clare Kramer and Charlie Weber didn't know about the link between Glory and Ben until the episode it was revealed.
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. Sarah Michelle Gellar Gellar enjoys figure skating in real life.
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.
Subverted with Alyson Hannigan. She requested that she not sing too much in "Once More With Feeling" because she hates her singing voice. But upon seeing the episode, she was surprised at how good she sounded and regretted not singing more.
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.
Jenny Calender's actress, Robia Lamorte, had become a born-again Christian by the time she reprised her role as a guise for The First Evil and didn't take kindly to it, seeing it akin to playing Satan.
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.
Some actors were a full decade older than their characters. In an extreme example, Charisma Carpenter was 26 when she played 16-year-old Cordelia.
Averted by Dawn and Faith, who are both the same age as the actresses playing them, Eliza Dushku was even legally emancipated from her parents so she wouldn't be limited by child labour laws. Dawn was likewise made a couple of years older to match Michelle's age, since she was originally written as a preteen. Mercedes McNabb (Harmony) was also sixteen when the first season was shot.
Dyeing for Your Art: According to Joss Whedon, all three lead actresses had the same brownish auburn hair colour when they were cast. Sarah Michelle Gellar eventually bleached her hair blonde for Season 2, Alyson Hannigan went fully redhead, and Charisma Carpenter darkened hers.
Kristy Swanson played Buffy in the movie, but Sarah Michelle Gellar took over for the TV show because Kristy was thought too old to play a sixteen-year-old at the age of twenty-eight. Likewise Donald Sutherland played Merrick in the movie, and is portrayed by Richard Riehle in a flashback to Buffy discovering she's a Slayer.
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.
Nathan Fillion as the sadistic, satanic serial killer working for the Big Bad (If you take the First Evil to be Satan and that Caleb had killed people For the Evulz before meeting it).
Playing Gertrude: Charisma Carpenter is older than Robia La Morte, who plays her teacher.
Production Posse: Several actors who had starred in Firefly had smaller parts on both this and Angel. Notably Nathan Fillion as Caleb. Jonathan M Woodward played Holden in "Conversations With Dead People" and Tracy in Firefly's "Message In A Bottle" (as well as Knox on Angel).
Kristine Sutherland announced she wanted to leave the series at the end of Season 3. Joss Whedon compromised and featured her in only five Season 4 episodes. She agreed to come back for Season 5 to be killed off.
Halfrek was meant to appear in more Season 7 episodes, but Kali Rocha was committed to a play. In fact she filmed all her Season 7 scenes in one day.
Shrug of God: Joss Whedon was originally shocked at people shipping Buffy and Faith. But when one fan had him look at her website, where she analysed all the Les Yay scenes between them, he later said the fans were spot on about the subtext between the Slayers.
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.
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.
The scene in the pilot where the Master blinds a vampire and says "you've got something in your eye" was added to fill time.
Kali Rocha was cast as Halfrek in Season 6 after having briefly played Spike's human love interest Cecily in a flashback. Knowing that eagle-eyed viewers would notice, they wrote in Halfrek saying "William?" when she sees Spike.
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.
Trolling Creator: David Fury wrote in many scenes of Buffy laughing just to troll Sarah Michelle Gellar, who hates laughing on camera.
The entire first season was shot before the pilot had even aired, so some of the scenes were re-shot. Buffy's "it's my first day" speech to Giles was actually the last scene filmed, as they decided that Sarah Michelle Gellar's originally performance was too aggressive.
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.
Ms Calendar was originally written as 'Nikki' instead, but they changed it to Jenny to avoid confusion with Nicholas Brendon (who was called Nicky by everyone on set). This was apparently no longer an issue by the time they wrote in the character of Nikki Wood.
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.
Tucker from Season 3's "The Prom" was set to be a member of the Trio, but his actor wasn't available. So they created Andrew as Tucker's brother, with a Running Gag that no one remembers him.
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. But when he saw the chemistry between Amber Benson and Alyson Hannigan, he made them a couple.
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. Seth Green had been cast in a film whose production was suddenly pushed forward, requiring him to be written out sooner.
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.
Xander was supposed to be revealed as Glory's host, tying in to his declaring at the start of the season that he's "through being everyone's butt monkey" and ultimately expelling her himself.
Throughout Season 7, Joss had terrible trouble deciding if Principal Wood would survive the finale. He describes the result as the episode just happening to be filmed on a day where he was leaning toward Wood surviving.
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, as well 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).