Since I am back to writing my new novel, and since I actually know the ending of this one (well, sort of), I have been thinking about the middle. Beginnings are easy for me. The first three or four scenes usually come to me along with the basic idea, wrapped up like a Christmas package, and I write them very rapidly in a few days. When I get to the end, it also tends to go fast, since by the time I'm two-thirds or so of the way through, I know the climax. (If I don't, the book is really in trouble.)
That leaves the middle.
How to get from C or D (we're way past A) to, say, V or W? This is where I think about the three Rs: reveals, reversals, and raising the stakes. This is, for me, what creates the middle of the plot.
Reversals: Who is doing well and about to get a pie in the face? Who is struggling mightily and might have some unexpected aid, even if only from his own inner resources? Whose careful plan is about to blow up in his face? Who gets a sudden (but foreshadowed) ally?
Reveals: What does the protagonist discover that he didn't know before? What do I show the reader that she didn't know before?
Raising the stakes: What else hangs on the outcome of this conflict besides what is already there? How does the prize get bigger or the cost get higher?
Okay... I just got an idea. Back to work.