According to Stefan Mumaw’s book, “Chasing the Monster Idea,” one of the key questions to ask if you want to find out whether your idea is a “monster” rather than just good (or bad) is, “Does it create an experience?”
I’d say “Alien,” which made me jump out of my seat when that thing exploded out of poor John Hurt’s chest…the ending of “Sixth Sense” which I didn’t see coming and was a topic of conversation for a while…and to be more general, any movies that make me wish they would just keep going for a few more hours (like “Sideways”) or movies that keep me thinking about them for days afterward (“Gone, Baby, Gone”), or ones (the “Bourne” films) that are a good thrill ride while they last even though they may be forgotten pretty quickly afterward.
There are a lot of ways to make your movie an experience but I think it helps to have one in mind as you’re writing. On the one I’m writing at the moment, my goal is to make people think about what they want to leave behind when they die, and maybe to feel a little nervous about the prospect.
If you haven’t thought about it already, consider what experience you want people to have. One way to focus on this is to write the review quotes you’d like to see when your film has been released–”A thrill ride that keeps you on the edge of your seat,” or “A hilarious look at parenthood that also makes you think,” for instance. Then, as you write or rewrite, make sure you deserve those quotes.