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.
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 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.
Sarah Michelle Gellar disliked Season 6 because of the dark tone of the storylines, leading to her ultimately leaving the show (ending it completely) after the seventh. She also wasn't a fan of the extensive preparation that had to be done for the Musical Episode. This is especially ironic because Word of God says that Sarah was one of the people continually lobbying to do it in the first place.
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.
Willow's "magic addiction" arc was disliked by both Alyson Hannigan and Joss Whedon himself; the latter added a scene in season seven's first episode where Giles explicitly states that magic is not addictive, and it's explained that Willow's actions were actually due to her not using magic.
James Marsters was so disturbed by Spike's Attempted Rape of Buffy that he had his contract updated so 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.
Emma Caulfield was a little annoyed at Anya's death when she saw the completed series finale. She had volunteered for the character to be killed off in the final battle but didn't realise it was going to be so sudden.
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.
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.
Darla was reportedly written out of the first season more quickly than intended because of conflict between Julie Benz and the regular cast members, especially Alyson Hannigan. She however mellowed out enough for Joss Whedon to bring her back for a couple of guest spots, and a recurring role on Angel.
It has been said Alyson Hannigan and Sarah Michelle Gellar, once close friends, were not speaking to each other by the time the show wrapped, with the former blaming the latter for putting her out of a job in a post-series interview.
Looping Lines: Anthony Stewart Head had to loop many of his lines as Giles in order to incorporate the character's mild stutter. This was such a hassle that Head declared he would never play a character with a speech impediment again.
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.
Post-Script Season: The fifth season ended with Buffy dying heroically to save Dawn and, by extension, all reality; the episode even ended with a shot of her gravestone ("She saved the world. A lot."). It was picked up for two more seasons on a different channel. Even after the series was cancelled for good, it received a comic book continuation starting with Buffy: Season 8.
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.
Serendipity Writes the Plot: When people kept telling Joss Whedon that the dialogue was what made the show great, he said "That's the only thing really great about the show? Fine I'll do an episode with no dialogue." - thus "Hush" was written.
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 cast would have been saved from many a scrape if they'd just had cellphones. Quite a few episodes use a character being in peril and unable to contact Buffy as a plot device. This wasn't a big deal in the earlier seasons, but the show hit it big just as cellphones were starting to become mainstream, so after a few years it began to seem rather odd, especially since the cast was full of teenagers (later, young adults), the group most likely to carry a cellphone. This was lampshaded at the start of the final season (in September 2002) when Buffy gives her sister Dawn "a weapon" to help protect herself, which turns out to be a cellphone. From then on most of the cast had cellphones — although ironically, they hardly ever needed to use them, since that season also saw every single character move into Buffy's house. One episode reveals that Giles does indeed own a pager, joking that they should page him if the apocalypse happens when he's not around.
Two notable examples from the second season: when Angel loses his soul and reverts to evil, the information that Willow needed to restore Angel's soul was stored on a 3 1/2 inch floppy disk that fell between desks. Earlier, Joyce's boyfriend Ted, who worked for a computer company, curried favor with Willow by giving her freebies from work, including a new hard drive with a gargantuan capacity of 9 gigabytes — a tad more than a $15 USB drive could hold little more than a decade later.
None other than Michelle Trachtenberg says she experienced this when she became a writer. Having been so used to her time on Buffy, she struggled to make her stories work in the age of cellphones and widespread internet.
"Buffy lived in a world where social media and cellphones weren't a part of the story. You couldn't Google "blah blah blah demon." You had to go physically to the magic shop and find a book. You cannot re-create that today."
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 Harvest" 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.
Joss joked that he was going to kill Anya off in "The Gift" - when she pushes Xander out of the way of some falling bricks - but Emma Caulfield kept moving too much in the final scene with all the survivors.
Too Soon: "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.
Tuckerization: Riley's last name Finn was taken from Marti Noxon's dog.
Unfinished Episode: 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.
Unintentional Period Piece: The early seasons don't so much scream "1990s" as they beat you with a sledgehammer labeled "1990s" and perhaps teamkill you with a tank. The effect gradually fades as the seasons progress, though. The same is also true of Angel, on an accelerated scale.
Writing by the Seat of Your Pants: Angel's extremely vague writing (plus comfortably standing in partial sunlight a couple times) in the first few episodes was because the crew started out with basically no idea what the deal was with him. His name was actually a holdover from one early idea that he was a literal angel who'd been expelled from Heaven and was trying to earn his way back.
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 the 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).
Creator's Pest: While Joss Whedon loved Kennedy despite the fans' hatred of her, the other writers grew to hate her as much as said fans.