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The ending is the last thing an audience sees. If it's amazing, they'll remember the movie as a wonderful experience. If not, they'll leave the theater disappointed.
There are three kinds of endings. Happy endings, downer endings, and bittersweet endings.
Happy ending: Boy gets girl (You've Got Mail), justice triumphs (The Verdict), and everyone lives happily ever after. We leave the theater feeling good, our hearts full of hope for humanity and ourselves. Because audiences love happy endings, Hollywood loves happy endings.
Downer endings: Boy gets girl but he loses something even more valuable (Body Heat), evil has destroyed good (Chinatown). The world is a mess, and we leave the theater sobered. Sometimes hardship elucidates life, and there have always been successful movies with downer endings.
Bittersweet endings: Girl loses boy, but she wins something more valuable (My Best Friend's Wedding). Or good triumphs over evil, but only because the hero made a personal sacrifice (Casablanca).
Endings must resolve the story question in a clear and unambiguous way. If the question raised in Act I was, "Will the boy get the girl?" your Act III has to answer with a yes or a no. Not a maybe. Even in a bittersweet ending, the answer is either "yes, but" or "no, but."
The ending has to be set up step-by-step. It can't come out of left field. It should be hinted at throughout the film. It should feel inevitable, as if that's the only possible way this story could have ended. It should make sense. But at the same time, the ending needs to be surprising.
What? How can an ending be both inevitable and surprising?
Look at Thelma and Louise again. Everything in the story points to Thelma and Louise not being willing to give up their freedom. Their whole journey has been about escaping from society's constraints. Are they really going to allow themselves to go to jail? No! They have to get away! That's the inevitable part. But the way they choose to get away—by going over the cliff and facing their death on their own terms—that's the surprise.
Once you've written your ending, go back through Acts I and II and verify that every twist, event, and revelation in Act III is properly set up. This is what will make your ending seem inevitable, and therefore true to the story. It will feel right.