I am thinking a lot about middles of novels right now, as I am just embarking on the middle of a first draft. The middle of a book is notoriously the place where the story slackens and readers find themselves slogging along, laboring to make headway through your book, almost fighting against you to make it safely across the swamp that is the Second Act of your story.
I think that there are a couple of things that make middles sag:
1. The story is too simple and the middle 100-200 pages are, frankly, padding (as Nick Hornby says, you're just keeping the reader's left and right hands apart).
2. The story is suspended for a few hundred pages while the writer digresses into discussions of his pet subjects, which have nothing to do with the story you're reading.
3. The plot becomes episodic, one similar conflict resolution after the other, and despite the author's imaginative new dangers to throw at the hero, the story feels repetitive (and dull).
4. The plot, the characters, the story are all sort of wandering around and going nowhere in particular for 40,000 words.
Now, consider that numbers 3 and 4 are actually symptoms of number 1.
In number 3, the story becomes a long series of episodes because the writer is attempting to show growth or progress or something, and it's all just mechanics: a dramatized list to prove that the hero is ready for the climax in Act Three. Or, the author is blindly following the maxim to "put your protagonist in a tree and throw rocks at him." But it's not a story, it's just the hero doing whatever the story's equivalent of pushups is. There is no real forward motion, there's just a lot of running in place or in circles. That is rarely interesting for long, and shows that (as I claim) the story is too simple to justify its length.
In number 4, the writer has no idea what to do and has to fill in space until it's time for the climax, so (because the story is too simple) the characters just mill about waiting for the bell to ring and let them back into class, as it were.
So really there are, as I see it, only two real reasons that middles sag: the writer decides to digress and leave the story behind for some length of time, or the writer has not got a strong enough story to fill the length of a novel.
Both of these problems are problems of structure, of the way the story itself is conceived by the writer. To fix Problem Number 2, my answer is Don't Do It. Seems pretty straightforward. But then you don't have a middle of your book, and you're back to suffering from Problem Number 1.
I pause to insert the idea that another possible problem with the middle of the book is that it's simply badly written and even if the story is well-structured, the telling is not well done. That's a whole other (and larger, sadly enough) topic. So back to where I was.
Problem Number 1: The story is too simple.
I see, in my wide travels as intinerant internet reader guy, a lot of early novels that have sort of one-dimensional stories. By that I mean stories where there is a problem put before the protagonist and they doggedly push on to solve it. There is, essentially, only one thing happening in the story, and the story operates at a single level: the level of the primary action of solving the problem. This means that these stories lack depth, usually, and they also lack variety and suspense.
They lack variety because there is basically one action: the protagonist moves toward a goal and encounters resistance. Even in those books where the protagonist gets the quest at the end of Act One, gathers strength during Act Two and then has a dark night of the soul before the climax in Act Three but suffers through the epiphanic moment to make the triumph possible (that is, books which follow the Campbellian Hero's Transformative Journey), you still have just the one real action, of the hero implacably moving against the antagonist. You throw in subplots all you like, you've still got a one-dimensional story going.
These stories lack suspense because the meaning of the actions are generally clear by the beginning of the Second Act. Yes, the story question ("will Luke destroy the Death Star and become part of the Rebel Alliance and clear up that skin condition?") remains unanswered, but if that's the only question the reader has, the suspense will turn to mere waiting during that long middle stretch. There is no mystery behind the story question, no deeper unknowns than the outcome of the primary conflict. That amount of suspense is not enough to carry a whole book.
What you need, then, is more variety to your story, more mystery and more layers of suspense. How do you do this? By making the story more complex. I am going to suggest a general framework for this, and it's a very old and reliable framework that's been around for centuries.
I call it the Inner and Outer Conflict Structure, and I have come to lean heavily on it because it works, it's flexible, it's generally invisible to the reader and it's applicable to all genres of fiction including literary.
It works like this:
You already have a story with a conflict (if you don't, then you don't have a story and this isn't the post for you). This conflict is either an external (Outer) conflict (Luke versus the Empire) or an internal (Inner) conflict (Gogol versus his idea of his parent's expectations of him, which is to say, of his expectations of himself). What you need to do is figure out which type of conflict you have in your story, and then come up with a conflict of the other type. That is, if your story has an outer conflict (George must kill the dragon), you need to find an inner conflict (George's father was saved by a dragon and George is, you know, morally opposed to killing dragons because he was raised with stories of the benevolence of dragons even though he has never, it must be said, personally met a dragon).
So you have your inner and outer conflicts. What you do then is build a three-part structure where the outer parts (Act One and Act Three) are the outer conflict, and the middle of the story (Act Two, natch) is the inner conflict. Like so:
Act One: George versus dragon (unresolved)
Act Two: George versus his feelings about dragons (conflict resolved during this act)
Act Three: George versus dragon (resolved)
When Act One ends, the outer conflict is still unresolved, and the reader doesn't know which way things will go. This is the unanswered story question, and is the primary suspense that will go stale if that's all you've got. So Act Two introduces the inner conflict, which is related to the outer conflict. This is variety, and the new story question adds more suspense for the reader. Also, the inner conflict is often more interesting to the reader than the outer conflict, which makes the reader perk up her ears and become more interested in your story as the middle of the book progresses. There's no reader boredom because there is a whole miniature drama in the center of your story, and this inner conflict must be resolved before the outer conflict can be. George can't commit to battle (or not battle) against the dragon until he works out how he feels about dragons in real life, once he has some exposure to the ways of real dragons. If he decides that dragons are as he thought, peaceful but misunderstood beings, Act Three isn't going to be a bloodbath. If he decides that dragons are actually dumb, violent beasts and his father made up the story about being saved by a dragon to hide a crime he'd committed, then maybe George will sharpen his lance and go slay a large reptile. Either way, the story has become much more complex, much deeper, possibly more meaningful and certainly it will be easier to write the middle when there's really a middle to be written.
If this is all obvs to most of you, then I apologize. If this is new stuff, or if you disagree with/see flaws in this version of the 3-Act structure, say so. If you have a different, but widely-applicable way of structuring your stories that defeats the devil of sagging middles, please share it!