Ann, a sharp reader, sent me a note about the non-sequitur of two previous posts...and I'm gonna swallow my ego & admit she's right.
I re-read my post & was chagrined that I didn't really answer David's question (apologies, David).
So I'm going to re-do the second post & answer his question as I should have. Because after all, darn it, I swore an oath when I became a reader. (OK, I made up my own oath, but the fact is that I'm sticking by it.)
The first post - Scorsese's one liner was unusual for the very fact that the attractive one line of dialogue embodied the movie and AT THE SAME TIME could easily be the logline. This is very rare & doesn't happen in every script. That's why I was so excited.
The second post - David asked whether this one line of dialogue should focus on theme or plot.
[Here's where I went off on a tangent in post #2. I didn't focused on the one liner being dialogue -- I focused on a one liner being a logline.]
On further reflection, I'm gonna say both, if possible. ex. The "man or beast" line from Shutter Island sums up the struggle and the arc all in one.
But if I HAD to choose, I'd probably say theme. To me, the theme tells me your idea - where you want to go, what you want to do, what you're aspiring to reach, i.e., the arc. The plot I can figure out later, as long as I know the germ of the idea.
Examples of theme:
ex. Tootsie - When a man is forced to pretend to be a woman, he experiences a world he's never known
ex. Jaws - A man eating shark menaces a small town (man vs. beast)
ex. ET - A family pulls together to help an alien who's landed on earth to return home