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.
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.
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.
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.
Dawson Casting: 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. Similarly, there's 22-year-old Kristy Swanson playing high school senior Buffy.
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 labor laws).
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.)
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."
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.
Wag the Director: According to Whedon, Donald Sutherland was under the impression that he was the star of the film. And rewrote all of his dialogue.
What Could Have Been: Whedon wanted to put Eric Balfour in the opening credits for the first two episodes so that his death would be a surprise, but literally didn't have the money for two sets of credits. He got to do it with Tara six years later.
When the show was being pitched and the WB were interested, they considered giving it a timeslot for the 1996-97 season (which would likely have resulted in a full 22-episode season 1); it lost this battle to Seventh Heaven, becoming a Mid Season Replacement when Savannah was cancelled in early 1997. In hindsight, had the WB execs. chosen differently both shows would likely have turned out different - among other things, Alyson Hannigan had not yet been cast; the Master arc would also have been effected, with an extra 10 episodes and the possibility of the Annointed One not being pre-emptively killed off; etc.
Darla was originally supposed to be killed in the end of "The Harvest" when Willow hurled holy water at her, but a more interesting plot for episode seven ("Angel") was needed so they kept her in the show.
James Marsters originally auditioned for the role of Spike using a Texan accent for the character. He and the writers eventually decided that this was not a good fit, and so instead of a cowboy, Spike became a limey.
Spike actually uses this Texan accent in Season #4 when he's attempting to convince people that he's an old friend of Xander's. It's hilarious.
"Normal Again" was originally more of a Rage Against the Author episode, with the world of Sunnydale crumbling due to Joss Whedon suffering writers block.
Bianca Lawson (Kendra) originally auditioned for the role of Cordelia Chase and won it, but she was opted to take a role in the sitcom Goode Behavior instead. Producers liked her so much that she was introduced on the second season as Kendra.
Buffy's bazooka in "Innocence" was originally a tank in the script.
Oz was originally supposed to be killed by Angelus in "Passion", but his character had a lot of potential, so the decision was made to bump off Jenny instead. In turn, Jenny was supposed to have died in an earlier episode, "Innocence", presumably in place of Uncle Enyos.
Warren was only intended to be a lackey on par with Jonathan during Season 6, working under Tucker Wells (from the episode "The Prom"), who would have been the real villain of the season. However, actor Brad Kane was unavailable to reprise the role, so Andrew Wells was created to be Tucker's brother as a substitute. Warren was likely promoted to main villain because he ended up being the most unpleasant of the three — Andrew was an amusing Ineffectual Sympathetic Villain, and Jonathan was generally well-liked by the audience by that point.
Amber Benson was slated to return in season seven. However she knows just how cruel Joss Whedon would be to her characters and to the fans, and declined the role. There is, in fact, a script for "Conversations with Dead People" that has Tara in place of Cassie talking to Willow, and it's exactly as cruel as you would expect.
An Animated Adaptation was slated to focus on the in series Cosmic Retcon with Dawn appearing throughout the first four seasons. One idea was to have Angelus attack her instead of Willow. You can get details on the trope page as well as watching the four minute trial footage.
Joss' original concept for what eventually evolved into "Buffy" was called "Rhonda the Immortal Waitress".
If Seth Green hadn't left the show, everything from season 4 and up would likely have been very different. Oz would have remained with Willow, Xander would then have likely been the one to come out as gay considering Joss Whedon wanted either him or Willow to eventually be written as a homosexual, which also means that Anya would have either been written out or given a very different role than the one she had in the series while Tara may never have existed. Come season 6, it also would have meant that Oz would have wound up being the one killed by Warren.
The Anointed One was going to be the Big Bad of Season 2, but Andrew J. Ferchland's growth spurt made it impossible for him to pass as an eternal child, so Spike and Drusilla were created to take his place.
The vampire Sunday from "The Freshman" was originally going to be mentioned as a Slayer who had been turned into a vampire, to explain why she was powerful enough to defeat Buffy in their first fight.
Angel was originally conceived as nothing more than a weird guy who occasionally showed up to give Buffy vague advice. There were several ideas for his true identity tossed around, including that he was an actual angel who needed to complete a certain number of good deeds before being allowed back into Heaven (which would have made him amusingly similar to Castiel from Supernatural), before his being a vampire was decided on literally while writing the episode with the reveal. This explains the severe Early Installment Weirdness with him until that point, as he had to be written very vaguely to fit whatever the writers came up with.
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 Lank plays a vampire lackey to Harmony a few season before playing Tucker's brother.