And I usually do start with some kind of meta in my head, like, "What do I want to say about these characters and their relationships?"

Yes! I'm also interested in using the relationships of the characters to reflect or comment on the Big Themes of the series and my own meta-commentary on those themes, both the ones Joss intended to put there and the ones he probably didn't.

But while that can be hard work, it's not labor in the sense I think TKP means. When I can get everything to come together - plot and theme and character arc and humor and the all-essential vampire porn becoming greater than the sum of the parts - it's glorious.