Thanks, Laura. I should add, by the way, that I agree entirely with Annie and LabRat: yes, we use snippets of dialogue in fanfic all the time, and we tend to assume that people know where they come from. So that's not at all a problem, and I should have said so initially. I didn't, though, because it seemed to me that you didn't even think you were close to the script - and now that you've posted your extract, as Pam and Rat have said, you have absolutely nothing to worry about.
Yes, ff.net is notorious for stupid and even downright rude comments from readers - either ignore her or post back asking her to show you exactly which lines are straight from the episode (and which episode), because you know that they're your own words.
Wendy
