It's not a problem, Jenn. If you're using lines of dialogue from the show in a story, simply put in a couple of lines in an author's note at the start to say so. That's all that's ever been necessary.
There's no danger of you being charged with plagiarism within the fandom you're writing in, because the main issue with plagiarism is someone trying to claim authorship of someone else's words. No one in your fandom is going to fall for that one - they know those words off by heart.
So there's never any danger that they could mistake something from the show for your own work, generally speaking. For that reason, many authors often don't bother with a credit because they take it as read that anyone in their fandom reading the story will be able to tell the difference as a matter of course.
But it's handy to make it clear anyway, just to be nice. Especially if you're a little worried about it. Bob's and BJ's examples are good templates. And I know it wasn't part of your question, but dialogue from sources outside of LNC - books, plays, movies, TV shows - always need to be credited, simply because they may be less familiar to your readers than LNC dialogue and it's best then to make it crystal clear that certain parts of your story aren't your work, so that there are no misunderstandings.
Using lines of dialogue from the show is an accepted part of fanfic - if you're tackling an episode rewrite you practically can't avoid it - no one will have an issue with you doing so. And in general legal terms it's no more a breach of copyright than using the characters etc from the show to write fanfic in general.
So have at it, when you need to. You'll be in good company with millions of fanfic writers before you.
LabRat
