As I get into reading the longer stories, I find that premise matters more and more. The difference between a very good story and a great one often falls to Premise.
Premise has a couple of different meanings. A lot of people say premise when they mean concept or even "plot summary." But technically premise means "meaning" or "theme," especially in Hollywood. (Ever see the movie State and Main? The characters go around telling each other that the premise of their schlocky big budget Hollywood movie is "purity.")
There are even some who feel that the premise should be like a thesis statement, a well developed statement of the lesson of the story. That's a useful thing to have when you're taking a meeting with people about a script, but I think the real important aspect of premise is really the more vague meaning of Theme.
You know how when you're noodling around with an idea, and it's just a bunch of loose scenes and characters until this magic moment when it becomes a story and you can write it? I think that moment is when you've discovered the theme. I don't mean consciously. It's just that the story takes on meaning and you know the real struggle of the protagonist, and you may even have an idea of a good title for it.
A mystery might be about justice, but it might also be about courage, or loyalty. And different characters will represent different aspects of these. A character with great physical courage may be a moral coward. Someone who has never thought about courage may stand up at the right moment on a small thing and make a difference. Someone who is not meant to be courageous may be inspired to make a terrible mistake by following the example of a hero.
The premise doesn't have to be anything that big. It can be about the relationships between sisters, or pride in your work. Whatever it is, it can inform the overall story in so many ways. It can make the story deeper and give you more to work with. It can give you a focus for all the little details that you fill in around the main plot.
And it helps you find the RIGHT ending.
The movie Casablanca was scripted with four different endings. They didn't know which one they were going to go with right up until they shot the ending. But it was clear they had the right ending as soon as Bogie said, "It doesn't take much to see that the problems of three little people don't amount to a hill of beans in this crazy world."
The whole of that movie was built on the theme of survivors. The sweep of history was destroying the lives of everyone. Nothing that mattered in an ordinary world mattered at all any more. Every moment of that movie was about the little people surviving and doing whatever they had to do, some positively, some negatively, and because of that, the real moral of the story was able to rise naturally from it. The survival of the world depends on us getting it together and putting our personal needs aside.
So when you find yourself losing direction, and you feel your story isn't driving along properly, think about premise. What ties these characters and their struggles together? Sometimes you can bring things back together by thinking about how a character or a scene reflects or contrasts the main character's problems or issues.