Necessary and Sufficient
Back in high school, I had a marvelous history teacher who made a point of going into more than memorizing dates and names and places. One of the key things I took away from that class was the concept of necessary and sufficient causes, and the difference between them.
Necessary causes are the things that absolutely must happen in order for some event or change to occur. In terms of plot, if you want to write a classic murder mystery, there has to be a murder. Sure, there are all sorts of other crimes, from theft to blackmail, that you could write about instead, but they won't be murder mysteries. Without the murder, there won't be a story.
But necessary causes are not always sufficient all by themselves. In order to have that murder mystery, you need the murder, but just the murder isn't enough. If the murder is never discovered, or if the murderer kills both the victim and himself at the scene of the crime, there's no mystery. (Which is why most people who pick up a murder mystery that opens with a murder-suicide will immediately assume that the real murderer has faked the evidence - because if this really is a murder-suicide, the story isn't going to be a murder mystery.) The story needs something else. Most of the time, the mystery is "who did it?", but sometimes, everybody knows who did it and the mystery part is in figuring out how they did it (and proving it enough to arrest the murderer).
Necessary causes are specific and immutable: without X, you don't get Y, not ever. Sufficient causes can be many and varied; they come in an assortment of different numbers and combinations. One or two big events can work, but so can a lot of little things that add up. The only requirement is that there are enough of them to tip the balance so that Y actually happens.
Plots and stories are about the sufficient causes - all the different ways of getting from here to there. And that can be a problem. A lot of people are quite good at figuring out what the absolutely necessary things are, and making sure to include them. Often, though, they're not so good at going beyond that. They put in the necessary elements, and they may even realize that they need a bit more than that…but they don't look closely at just what it's going to take to be enough to tip the story in the direction they want it to go.
The whole thing is made more complicated by the fact that exactly how much is enough to justify a plot twist or story event can vary wildly from reader to reader. Some people take a lot more convincing than others. Furthermore, there can be more than one thing in a story that needs sufficient cause before the whole story is believable. I had a horrible argument once with a gentleman who didn't understand why his beta-readers didn't believe his fantasy adventure. His central plot developed quite logically, after all … but a lot of the background politics were, at best, thin; at worst, they looked much too convenient. There weren't sufficient reasons behind them to make the readers believe them, and therefore they undercut the believability of the main plot. The background didn't matter to him, so he thought he had plenty enough to make the story work; unfortunately, his beta-readers had different standards for what was enough, and he hadn't met them.
In other words, it's necessary for the main plotline to hang together, but often it's not really sufficient. Even if you leave aside things like interesting, well-rounded characters and a writing style that doesn't go thud-clunk, which are also necessary for many readers, a novel has subplots and background and an assortment of other details that have to make sense. An author who wants to write a stripped-down, minimalist sort of story is mainly going to pay attention to the necessities and only the necessities - but in order for that to be enough to satisfy, that author is going to have to pay extra attention to what is necessary at all the different levels of the story, because there aren't going to be very many places to work them all in.
An author who is doing a more usual sort of story still has to get all the necessary stuff in - it wouldn't be called necessary if they could leave it out - but there are more possible places to put those important bits. It is even possible, sometimes, to let them be implied instead of stating them (though that can get tricky, because no matter how clear you try to be, some people will miss picking up on it).
