There are more paces than just "fast". Let's start there. When authors talk about pacing, they often mean, "How can I make this book go as fast as possible?" The first question, however, should be, "What pace will help me give my readers a better experience?"
After all, readers who like a Henry-James-like contemplative approach don't actually WANT your book paced like The DaVinci Code.
So what pace is right for your book? More on that in another post, but right now I want to talk a bit about a technique that can help create a different pace. That's what I call the "BAR" method.
Action (or event).
Reaction (or emotion/aftermath).
This is a typical sequence for a pacing/event unit. (Pacing is all about how many events you have and in what period of time/pages.) We will, of course, have a bunch of these in a book, maybe 15 in a fast-paced book, or 5 in a leisurely-paced book.
First you do the buildup, whatever decisionmaking, investigation, exposition, and/or consideration has to happen before the action or event. Like if Sadie is (in the coming action) going to find a clue, the buildup might be where she figures out what she needs to know, and decides to risk arrest to get it. This is about anticipation, preparation, consideration.
Second, there's the action or event-- what happens. This is the most essential part of pacing as it's how we calibrate pacing. A thriller is "fast-paced" because it has a lot of actions and events coming quickly. A more contemplative or leisurely paced novel will have fewer actions in the same amount of time. The action or event is something that a character does (or happens to the character) that changes the course of the story. So this would be where Sadie breaks into the office and steals the old typewriter and finds the letter on the old typewriter ribbon. (Typewriter ribbons were really cool clue-locations.)
Third, there's the reaction or aftermath. That's where the character (and/or the world of the story) reacts to the event, shows the emotional or actual change because of this event. So Sadie gets back to her apartment and reads the typewriter ribbon and realizes it implicates her own mentor, and she is plunged into doubt and despair and can't decide whether to turn this over to the police, or protect her mentor.
Now those three elements can be each in a separate scene of more or less equal length, or separate scenes of different lengths, or just passages within a scene. But I think -- just speculating here- that if we want to make the pace of a book something or other, we can do this by manipulating the relative length of these elements.
Let's say we want a more methodical pace that's suitable for a police procedural. We might amplify the Build up section to show the careful consideration Sadie goes through figuring out what she needs to, why she needs this clue, whether she ought to commit a crime to get it. Or conversely, for a suspense thriller, we might want to expand this passage to build up the suspense, the danger involved, the threat to her life and career, the looming darkness that is the villain-- before she does what she has to do. Fear, suspense, threat have to be created in the reader by the build up to an event. But of course, that creates a slower pace.
Or maybe we want a fast pace for an adventure book. In this case, the build up might be pretty short, just enough to set the stage for the action. Then the action scene might be long, exciting, and richly detailed, very physical to give the reader a visceral experience. So in this case, there wouldn't be a lot of Sadie debating the morality of breaking in-- She decides to do it and just does it, but we get a full account of all the obstacles she encounters and the last-minute interruptions and the near-miss security guard patrol, and the slippery feel of the folder in her fingers.... Expanding the action passage to a greater length than the other two elements will lead to a more adventurous pace.
But then, what if we want the reader to feel emotion, to identify with the angst and triumph and despair of the character? Then we might spend the most time on the reaction or aftermath, where the character deals with the impact of what she's done or learned or experienced. Romances particularly I think often need long aftermaths to create the depth of feeling and the potential for interaction that deepens the love story. So in this case, Sadie might be rocked by her sense of betrayal by the mentor, remember all the times Mentor helped her do her math homework, and plunge into a despair because he is the only person she trusts, and now she might have to betray him. (And she has to call that hunky but arrogant police detective.... Romance!)
Anyway, no rules here, but if our story doesn't have the appropriate pace, it might not be a problem with the plot or characters, but rather just a matter of manipulating this BAR sequence so that the most important element to our pacing choice is emphasized by being longer than the other two. I don't mean making the action scene 15 pages long in a fast-paced book (long scenes will slow down the pace), but rather making it the longest of the three, so maybe we have a paragraph or two of build up, five pages of action, and two paragraphs of reaction.
What do you think? Look at your own stories. What sort of pace are you hoping for, and what kinds of passages or scenes go on longer?