Producers were reluctant to cast Amber Benson as Tara in Buffy the Vampire Slayer 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. While 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 was previously a regular on Jonathan Creek, which used this song as its theme.
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.
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 ironic 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 "Lover's 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.
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."
Nathan Fillion was fresh from Firefly. Whatever fan glee existed surrounding his arrival was crushed after Caleb's true colors were revealed five minutes in. People watching the show for the first time now, will likely recognise him as Castle.
Kal Penn, more well known as Kumar and Kutner shows up as a random college student in "Beer Bad".
By this point, D.B. Woodside, who played Robin Wood in the final season, is likely far more recognizable as Wayne Palmer from 24 (Woodside actually joined the cast of the series in Fall 2003, the following television season after Buffy's finale)
George the Janitor from "I Only Have Eyes for You" was played by none other than JohnHawkes.
James from the same episode played by Christopher Gorham aka Henry from Ugly Betty
Throw It In: The famous Mutant Enemy title card was reportedly scribbled by Whedon with only seconds to go until broadcast.
It has several variants through the series.
Graduation Day: Wearing a graduation hat
Amends: Wearing a Santa hat
Becoming 2: Says "I need a hug"
Once More With Feeling: Sings the "grr, argh" line in falsetto
Storytellers: Sings "We are gods!"
Chosen: Looking right at the viewer
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".
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.
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.
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. Jenny Calendar 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 likely would never have existed. Come season 6, it also would have meant that Oz would have wound up being killed by Warren had he and Willow still been together.
Buffy as a movie series, with a Hollywood budget and talent? Sure, we wouldn't have gotten what we have now, but just the thought...