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."
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, 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.
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 7th 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.
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.
Many characters stayed on longer than anticipated. Both Jenny Calendar and Joyce Summers (Buffy's mom) were supposed to die in the first season, but Jenny survived until Season 2, and Joyce until Season 5. Jenny only died because the fan response to Oz was so positive, otherwise he would have been the one killed by Angelus. And had he not left, Oz would have died instead of Tara, triggering her Dark Willow phase. Mayor Wilkins was only set for 2 episodes. Anya was to only appear in "The Wish", and after that was only supposed to appear once again in "Doppelgangland". Faith was originally signed on for only 5 episodes. Spike was to die at the end of "What's My Line?" but it was decided against. Spike was thus planned to exit the show after his one Season 3 appearance, but was brought on as a regular in Season 4 instead.
Another contributing factor in Jenny dying was that Robia LaMorte became a Christian and was no longer comfortable playing the part.
Angel was supposed to stay dead after the second season, but the WB network came to Whedon with the desire for a spin-off series, so Angel was brought back during the third season to set up his spin-off.
Search "Buffy Unaired Pilot" on YouTube to see a whole episode of What Could Have Beens answering questions like "What if Willow was fat?"
When given their characters, James Marsters and Juliet Landau were told they could use either a British or American accent. Marsters actually considered giving Spike a Southern accent, but Landau says that, given the "Sid and Nancy" vibe of their characters, Drusilla needed to be a Cockney. She admits that a Southern accent would have added interesting Blanche DuBois comparisons, though. Marsters originally auditioned for the role of Spike using a Texan accent. 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.
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.
Wesley was originally going to be an American Watcher.
From the very beginning, Joss Whedon planned to reveal that one of the main characters was gay. However, until Season 4 he had not decided as to whether this would be Willow or Xander, and subtle clues (extremely subtle) were placed for both characters in case his plans were able to see light. Ultimately, he decided to pick Willow instead of Xander.
Presumably this doesn't include wish-universe vampire Willow, who was not subtle.
Despite some popular belief, Tara was not necessarily introduced just as a Love Interest/Closet Key for Willow (though the timing is impeccable) - she was created because Willow was getting too powerful as a witch and outgrowing her initial "cuteness", so the writers realised that using her to make the villains sinister (and get captured/kidnapped) was no longer plausible. Thus, Tara was created to replace her in this role as The Woobie, as well as to explore Willow's "addiction storyline" that led to Dark Willow in Season 6 - Tara became Willow's Love Interest because Alyson Hannigan and Amber Benson had wonderful on-screen chemistry and the writers seized the opportunity presented to them. This leads into another WCHB - if Oz hadn't been written out of the show, would Willow and Tara have been written as lovers (before or after Oz's death led to the Dark Willow arc), or would they have been straight and Xander turned out gay/bi? Some combination of the aforementioned? Who can say for certain?
At one point Joss toyed with the idea of putting vampires in out-of-style clothing, to show they were mentally still stuck in the time they died in. This would have been an interesting contrast to a fashion-conscious teenage hero, but Whedon soon realized it would be impractical in the long run. Buffy still picks a single vampire out in a crowd due to his outdated fashion sense in one early episode.
Gail Simone was invited to write a season for the series, but was under exclusive contract with DC Comics, and declined.
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 original performance was too aggressive.
Angel was originally supposed to only appear in the pilot. In fact, the original unaired pilot doesn't feature him at all.
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.
The Master was originally supposed to have a beard and long hair before Mark Metcalf suggested the bald look as a homage to Nosferatu.
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.
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 "Angel" was needed so they kept her in the show.
When Season One was being produced, the Anointed One was planned as Season Two's Big Bad. However, Andrew J. Ferchland's growth spurt made him portraying a theoretically ageless vampire unrealistic. If this had happened, however, we might not have met Spike and Drusilla, and the beard wouldn't have been grown.
In the original script for "Innocence" Buffy didn't have a bazooka. She had a tank!.
"Killed By Death" was originally rather different: it involved some sort of hybrid senior home/orphanage, and it would seem like the kids were evil, except actually the seniors were, worshiping some kind of panther god or something.
The Glove of Myhnegon from "Revelations" was originally supposed to be a falconer's glove that summoned some sort of demonic bird; Gwendolyn Post would try to control it, but it would kill her instead. Apparently the budget would have only allowed the bird to appear for about 30 seconds, and it became a joke around the studio that this particular script was unfilmable. Hence the change to a simpler Shock and Awe power.
One storyline idea was that Buffy would have found Faith had hanged herself, unable to live with killing an innocent man. What the writers came up with instead was much more interesting.
Initially Wishverse Buffy was going to be decked out in more military style clothing, with a larger cross. The writers must have thought it would look silly so they went with a grittier, harder version of Buffy's normal dress sense.
Whedon intended for a while to kill Joyce at the end of Season 3, and by the time he changed his mind Kristine Sutherland had booked several other jobs for the following year, which is why she appears so little in Season 4.
In "The Freshman", antagonist Sunday was originally going to be a Slayer who had been turned into a vampire, explaining how she was able to nearly effortlessly defeat Buffy the first time they fought. Instead, she simply became an unusually savvy and powerful vampire. After huge amounts of fan speculation on the idea, a vampirised Slayer finally did appear in the "Season Nine" comics.
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.
In the fifth season they considered revealing that Xander and not Ben was actually Glory's alter ego — presumably that would have involved Glory's memory glitch spell affecting the audience all this time — and having him killed off at the end of the season.
At one point, Joss wanted Sunnydale to sink into the earth in Season 5. This happened in Season 7 instead.
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.
Rumors exist of a crossover idea for "The Gift" with Team Angel coming to help.
Tucker Wells (from "The Prom") was going to be the leader of the Trio in Season 6, and the interim Big Bad until he killed Tara and Dark Willow then killed him. This would have made particular sense, since in his one appearance he was rather callous about killing people and even darker than Warren (who was originally a weak-willed, somewhat amoral nerd with some Hidden Depths). Jonathan would've been the same pretty much, but Warren was going to be the passive follower (this suggests that Andrew's arc in Season 7 might have ended up being his). Since Brad Kane, Tucker's actor was unavailable to reprise the role, Warren was upped to Big Bad via Jumping Off the Slippery Slope & With Great Power Comes Great Insanity, and Andrew was created to serve the weak-willed-follower role (as Tucker's brother who caused a chaotic incident absolutely nobody seems to remember).
Originally, Amy was going to be trapped as a rat indefinitely, but then Joss Whedon got a letter from a fan who pointed out rats only live for 3 years, and Amy had been in rat form for roughly three years. Amused, Joss arranged for Amy to return to human form. Amy's actor Elizabeth Anne Allen later thanked that fan for helping her return to the show.
In "Smashed", Willow and Amy get harassed by two homophobic men, and punish them by using magic to strip them to their underwear and make them dance in cages. In the original plans, they would have used magic to force the two men to kiss each other. Joss Whedon vetoed the idea because he did not want to portray people's sexual orientation as changing in an instant and he did not want to portray same-sex kissing as a punishment.
In "Grave", Buffy would have fought the dragon that escaped in "The Gift". It was changed to earth monsters because of the budget.
There was talk about bringing Tara Back from the Dead in Season 7. At some point in the season, Buffy would get access to one wish that could alter reality. She would consider a number of options, including restoring Angel's humanity. Eventually she would return to Casa de Scooby to show her cute new shoes off to Willow...
Willow: You used the wish on shoes? Buffy: Of course not. (Buffy leaves, and Tara's standing right behind Willow...)
They could not get Amber Benson's schedule to work with the plot, though, so it was scrapped. There was also talk of bringing Amber Benson back as one of the manifestations of the First Evil, but she refused because it would be too cruel to Willow's character and the fans. (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.) The echoes of that plot could be felt in "Conversations With Dead People", which had Cassie appear and claim to be speaking for Tara.
A shooting script for the Tara version of CWDP did the internet rounds back in the day, with truly heartbreaking Willow-First!Tara dialogue (Willow with Tara but unable to touch her — think about it).
In "Conversations With Dead People", they considered having the First appear to Anya as Halfrek, or to Xander as Jesse.
Whedon discussed with Nicholas Brendon and Sarah Michelle Gellar the idea of Buffy and Xander ending up together at the end of the final season, which the actors reportedly agreed. Gellar has stated she believes Whedon's original intention was "was to put Xander and Buffy together. I really do believe that."
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.
Continuations and spin-offs
At the height of its popularity, an Animated Adaptation 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.
The proposed live-action Ripper spin-off, which would have been a Hellblazer-esque dark supernatural thriller starring Anthony Stewart Head as Giles, initially intended to be set during his bus excursion in the sixth season. Whedon described at as "Cracker with ghosts". It was reportedly intended to have been made in the UK as a co-production with The BBC. With hindsight it appears that the Beeb were never actually very interested as at much the same time they were considering the revival of their own biggest fantasy property. There were still occasional fan rumors of the show being made for a couple of years after Buffy ended, but Head signing up for Merlin probably put the final nail in the coffin.
Ripper would have introduced Giles' eternally young great aunts Lavinia and Sophie, played by Anthony Head's daughters Emily and Daisy. They later appeared in the Angel & Faith comic series as recurring characters.
Tim Minear was behind an idea for a spinoff in 2003 featuring Faith. He pitched as "Faith meets Kung Fu" and it would have seen her Walking the Earth on her motorcycle trying to find her place in the world and that Spike would appear. This was abandoned when Eliza Dushku starred in Tru Calling and Spike joined the last season of Angel. Ideas intended for the spin-off were later borrowed in small part by Brian K. Vaughan for his "No Future for You" arc in the Season Eight comics.
Jane Espenson has said that back when the series was nearing its end, "I think Marti talked with Joss about Slayer School, I assume there was some back-and-forth pitching."
Espenson revealed more information when she gave a talk at Ball State University in March 2003. The show might have used some of the Potentials, and other characters from Buffy, which might have included Willow. Espenson also revealed that Whedon did not think that such a spinoff felt right. It seems that the concept for "Slayer School" was never developed beyond a 'pitch' for a potential spin-off to replace Buffy.
A Spike movie was kicked around for a while. The idea was considered dead by 2006 and in 2012 James Marsters confirmed for 411Mania that he would not play Spike again: he felt he was too visibly old to play the character without having "to cheat". In a later interview in June 2016, Marsters clarified that he might reprise his role as Spike in the future, but only if technology had advanced far enough that it would be able to convince viewers that Spike had not aged. He stated that in 2004, when he was asked by Whedon if he would be willing to be cast as Spike in future projects, he replied that he would be able to portray him for up to seven years after the end of Angel. After this time had passed in 2011, Marsters believed that he was too old to play the part. Marsters' and Whedon's ideas became the graphic novel Spike: Into the Light.