This idea has long fascinated me and frustrated me. It fascinates me because of the vast amount of power that just a few words can have to change a meaning. Think about it. You can take a movie of any length, say two hours, and with the right word or sentence... only a few seconds long, you can completely the change movie. That is amazing. And it's a real testament to the power of words and of language. It's also the basis of "the twist."
What frustrates me is that this should be obvious to anyone who writes. Yet, so few scriptwriters pay attention to the importance of the details. They look for big themes, cool moments and symbols of whatever point it is they want to make, but they completely overlook the little things that trip up films. How many times have I seen a film that makes no sense, but which could be saved by the hero uttering one single line... one single line to explain why they did something inexplicable or couldn't do something obvious. How many characters could be given depth with a single additional line of dialog? How many could be kept from being ruined by avoiding that stupid line you never needed to say. As a lover of words, and an understander of the power of words, I find this very frustrating that people who make a living writing can't see these things... or won't bother.
Anyway, let's make a game out of this. And for this, I need to credit tryanmax. He mentioned a couple weeks back how you could completely change the meaning of Planes, Trains and Automobiles by adding this one line: "Marie is in the trunk!" Wow. LOL! Yeah, that would totally change the movie!
So lay it on us. Tell us how you could totally change a movie with one little line.