Who are David and Christian? Sooo cute that groo and Lorne were friends! :heart:
David Boreanaz and Christian Kane -- Angel and Lindsey. They were also friends of Glenn Quinn, and Christian was uncomfortable with the episode where he impersonated Doyle after Quinn had died.
On a related note, Kane auditioned to be Riley but Joss offerred him Lindsey instead.
Ryan Reynolds auditioned for Xander.
Nick win the role of Xander because of a joke he told about shawarma (now you know where that joke in The Avengers is from)
Sarah and Eliza never talked to each other about how to play each other in Who Are You - they simply watched film of each other extensively before filming.
Bad Horse from Dr. Horrible was originally an idea Ben Edlund had for a villain on Angel, but was considered too campy for the show.
The actor who played Ben in season five wasn't told for most of the season what his connection was to Glory.
Miracle Laurie, who played November in Dollhouse, auditioned four times for a role in Buffy, which she never got.
That's all I can think of right now.
EDIT: The original script of Five by Five called for the end fight to take place in the rain, but the idea was struck down because the show didn't have the budget for a rain machine. The week they filmed that episode, a freak rainstorm hit LA, allowing them to film the scene as originally written (I'm so glad they could - to me, that is the most powerful tear jerker in the entirety of both shows.)