I think it’s important that when we start a book, we focus on the writing and the story and not all the extraneous stuff like the market, finding an agent, etc. I know from my own experience that often times opening the door to all that other stuff can cause a sort of paralysis — a fear of making the wrong choices with the story that will doom the project forever.
However, at a certain point I do think it can be helpful to begin thinking about how a book will be packaged/pitched because that can actually significantly impact the reader’s experience. Specifically, I’m talking about being aware of what sort of information might end up on a book’s flap copy and where those events take place in the book. For example, the flap copy for Twilight begins with “About three things I was absolutely positive. First, Edward was a vampire.” So the reader knows from page one what Edward is which means the scene in which Bella whispers, “I know what you are” doesn’t have impact because she’s finally figured it out, it has impact because of the emotional implications of her confronting Edward with the information.
Likewise, we know from the flap copy of Hunger Games that Katniss ends up as a tribute. Therefore, the reader doesn’t want to spend hundreds of pages waiting for Katniss to reach that point — in the reader’s mind, this has already happened. So those scenes in the book about Katniss becoming a tribute aren’t focused on the “will she or won’t she” question, but they’re there for other reasons: to show the brutality of the system, to show her love for her sister, to demonstrate Katniss’s character. The “will she or won’t she” has already been answered by the time the reader picks up the book.
I read a book recently that had a similar setup in which a character had the possibility of being chosen for a certain task and she spends ages going back and forth on what decision to make. As a reader this became frustrating because there was no suspense in the answer — I already knew from the flap copy what the character would do and the scenes of her waffling didn’t add anything. They weren’t setting anything up for later in the story — they only existed to create a “will she or won’t she” tension that was already resolved for the reader.
Of course, when you start drafting a book you have no idea what might end up on the flap copy (and in many cases the author doesn’t have control over the flap copy for their books). But sometimes you can make a pretty good guess what will be on there. If your book is about a girl who is the first astronaut on a mission to mars, the reader knows that at some point (hopefully quickly) your protag becomes an astronaut and is accepted into the mars mission and the rest of the book is about what happens after that. Note that this is likely a different story from one in which a girl *hopes* to become the first astronaut on a mission to mars. That story would focus on the process of becoming an astronaut and the tension of “does she succeed or not.”
One of the main reasons I think this is important is to make sure that as a writer you’re not belaboring a point that will already have been resolved by the flap copy. Ultimately, this will just work to frustrate your reader. Of course the text of a book has to stand alone and not every reader will read the flap copy, but if your book is about a cheerleader fighting evil, don’t spend ages going through every stage of the cheerleading tryouts hoping to create tension in the question of whether she’ll make the squad. She will… or else the book wouldn’t exist. The reader knows this. So if you do want to show the tryouts, make sure it serves a different purpose for the book.
As an exercise, think of some of the books you’ve read or movies you’ve recently watched and read the flap copy/watch the previews and then try to figure out how long it takes to learn that same information in the actual story. How did that timing feel for you? Too fast? Too slow? Think about what drove you to pick up that book in particular: what did it promise you on the flap copy and how quickly were you satisfied? Notice how sometimes, if you give a reader information on the flap copy, you don’t have to spend ages giving them the same information in the book itself. Rather, you can hit that point and then move on into the meat of the story. After all, the flap copy gives the reader a promise but it’s the book itself fulfills that promise.