Or else, as some people will no doubt say, my inability!!
But, I think the extra effort pays off. The paragraph I worte last night, after struggling for hours, was as close to 'right' as it could be. Nothing happened , no drama, no angst, no violence, no sex, certainly nobody acting out of character, but it 'worked' and even if not many people ever read it *I* know it's right, that the characters are portrayed as close to they way they were on screen. And that is all that matters to me.
It would be so easy to write without the effort, to simply ignore the 'reality' of the characters, but THAT is the challenge ( to me) of fanfic. Writing within those tight confines of canon. I could have churned out all one hundred thousand words of The Shepherd in possibly four months, but the characters wouldn't be who they are. And I wouldn't have had the chance to develop Rebecca ( and Sara) as much as I have done.