This is about how the course of the show was changed by things the writers didn't plan, like the actors needing time of or wanting to leave.
This was inspired by me watching the end of s6 and being as bugged as i always am by the conversation between Buffy and Giles when Giles returns. Don't get me wrong, the part when they're both laughing hysterically cracks me up, the bit i hate is when they're talking aout Giles leaving and Buffy says it was the right thing to do. It wasnt. I heard (im not 100% this is accurate) they wrote Giles out for a while because Tony Head wanted to spend some time in England. The problem they then had to face is that Giles leaving at a time like that was out of character and a little callous. Buffy had just gone through arguably the most traumatic experience of her life and he left because, after a few months, she wasnt coping well enough? I was never satisfied with that explination and i think the fact that they needed a quick excuse for him to leave showed.
On the other hand it was circumstance that led to Angel leaving Buffy. He had become popular enough for his own show, and i think that was great for not only the fans who got Angel, but for BTVS. Staying would have made no sense for him, the way he left and the reasons he gave seemed exactly what the character would do. If he hadn't been given his own show would he have stayed, keeping Buffy in a relationship were they could never sleep together incase he lost his soul? the way he viewed their relationship and the fact he felt he was keeping Buffy from having a normal boyfriend, it would've been odd if he'd stayed despite of that. So circumstance worked in the shows favour then.
My final example is one i have mixed feelings on. Oz leaving. I loved Oz as a character, but prefered the willow/tara relationship so it's hard for me to say if i think his leaving was a good thing. Still Oz left because Seth Green wanted to go. Again i wasn't completely satisfied with the first reason he gave for leaving, basically because being a werewolf made him dangerous even though he had been a werewolf for ages before then (i know he put willow in danger in that episode which prompted him to go but i still dont think that was a good enough excuse since that was an isolated incident and usually he had an easy time controlling the situation) but the second reason, after he comes back and his jealousy over tara makes him attack her, i could accept as a valid reason for him to go.
So that's three ways circumstance affected the show, one for the worse one for the better and one im torn over. What do you think about how it changed Buffy? was circumstance good or bad for the show?