From my understanding, word for word would be plagiarism unless it’s just one, two or three words because it’s too short to fall under copyright and I’m pretty sure the Episode Guidelines state that we can’t have excerpts from film or television, but we can have at most 5 lines of song lyrics. I personally don’t see any issue with one or two lines that are word for word if it’s a reference and/or joke.
As for reporting… I would not report unless the plagiarism was heavy and consistent (as well as so obvious and recognizable) but to be honest I don’t watch many movies so I probably wouldn’t notice anyway. 
And yes, to me, the line between inspiration and plagiarism has the potential to be very fine, but varies for different authors. Some have an incredible talent to steer completely away from plagiarism and work solely on inspiration whereas others may have more of a low-self esteem/confidence in their own creativity resulting in a partial or complete reliance on plagiarism (not saying every author is either one, there are many variables).