Join Date: Jun 2001
Location: the states
I'm no expert, but what I remember reading is...
They break it as a team, but then write it individually unless there are multiple writers credited. However, when Joss reviews the finished ep, he'll either rewrite scenes or tell them to make some changes with suggestions. I'm not sure how he decides to do it himself or gives it back to the writer, but i suspect time may play a part in it. I know I've read that he basically redid BB&B, Passion and IWRY and I'm sure there were others.
If more than one writer is credited for the ep, they usually divide it up by act. I think Checkpoint was co-written by Jane and Doug and you can really tell who did what. I bet she did the part where the CoW is interviewing the scoobies...that's just classic Jane there. The exception to this rule is Flooded. Jane said on the Succubus Club that she did a draft, gave it to Doug Petry to edit which meant that he rewrote it his way and gave it back to her for editing. Then she rewrote it...and back and forth like that. She said it was a terrible way to write an episode because everyone wants there stuff in there.