Katie Holmes could've played Buffy.
According to a 2000 biography, Katie Holmes was offered the role of title character Buffy Summers. Of course she ended up on the network a year later, in Dawson's Creek.
Sarah Michelle Gellar auditioned to play Cordelia.
Casting director Marcia Shulman felt that Gellar was "too smart and too grounded" to play an outsider like Buffy. Shulman said she made a "fantastic Cordelia," but once the network learned Sarah Michelle Gellar was attached, they suggested she play the lead role instead, based partly on her prior work on daytime soap All My Children. Several actresses who auditioned to play Buffy went on to land other roles in the show, including Julie Benz, Mercedes McNab, Julia Lee, Elizabeth Anne Allen, and Charisma Carpenter.
Yes, Charisma Carpenter auditioned to play Buffy.
Brunette beauty Charisma Carpenter initially read for Buffy, before she was cast as Cordelia. Over the years, Carpenter's performance helped guide Cordelia through one of the strongest character arcs in the Buffyverse, from vain Sunnydale High snob to a much more complex and grounded young woman who never lost her edge. Not only was she a love interest and strong partner for Angel on the spinoff of the same name, she even ascended to full on goddess status. As the actress herself once told the BBC, "I'm extremely pleased that I wound up with the character that I have." The role of Cordelia was first offered to Bianca Lawson, who was later cast as Kendra.
The original pilot has a different Willow.
The name Willow Rosenberg, the high school brain and shy wallflower turned badass witch, is rightly synonymous with Alyson Hannigan to the millions of Buffy fans around the world. It's hard to argue against the idea that her casting was meant to be. Hannigan even married one of her Buffy cast mates, Alexis Denisof. But in the original unaired pilot, Willow was actually played by an actress named Riff Regan. Plenty of shows recast after a pilot. Star Trek, for example, was famously reworked.
Ryan Reynolds passed on Xander.
In a 2008 interview, Canadian actor Ryan Reynolds explained that while he loved show creator Joss Whedon, he wasn't ready to return to high school. "I had just come out of high school and it was awful," he said, with a non-network friendly expletive in front of the word "awful." Empire co-creator and The Trio coconspirator Danny Strong was among those who auditioned for the role of Xander, which of course went to Nicholas Brendon.