Bof...

What a gem we have here (as long as YouTube will allow us to have it here, that is!) - this is the unaired pilot episode of Buffy The Vampire Slayer! Complete with a different library set - and an entirely diferent Willow too! Willow - that nerdy brainy girl whose character de-evolves during the entire course of the series, from suburban good girl nextdoor down to a sicko with an interest in the occult and from there all the way down to a bonafide witch (the way nerds imagine them to be anyhow) with notable sapphic tendencies that she indulges as much as she does the damned craft?
The original actress cast in the part -seen here- was Riff Regan.
Perhaps she foresaw the whole debacle the character would take? Perhaps not... Perhaps she did too - and was unwilling to make all the necessary "adjustments" (kiss another female thespian repeatedly - have love scenes with her - bare some part of her anatomy - appear to embrace the darker aspects of her nature by espousing the craft; not necessarily another woman! And, worst of all, becoming utterly EVIL - as much so as the entities she was supposed to help fight off...!)
Fans of the show will say that Allyson Hannigan, who eventually did all that, is the better actress, one with all the necessary range to effectively make all of those "adjustments"... The truth is that her only range was displayed early on - when she played what she was evidentally not in real life; such a goody two-shoes! She was more "herself" as the show progressed and her character, in fact, regressed! Into darkness! Yuck!
Really, the only "good" substitution/recasting in TV lore was the discarding of Elizabeth Shepherd as Emma Peel - which gave us then the delectable Diana Rigg in the part!
Everything else - from April Dancer, the Girl From U.N.C.L.E. to Willow - they would have been better served with the original casting choice!
I don't merely "think so" either - I know so! ;)