Remember doing book reports in grade school? It was probably our first foray into essay-style writing, all based on what we had just read. Our first ones probably amounted to, "This book is about a lady. She has a horse. She loves the horse. They ride around the country..." Yadda, yadda yadda. We basically take a work, process it through our ten-year-old mind, and retell it in about one-hundred words. The teacher smiles and we get graded mostly on our spelling and penmanship (back when writing was a thing). As it turns out, knowing a story from this ten-year-old perspective helps (trust me; I'll get to it).
As adults, we primarily read book reports in the form of reviews, but we look for the ideas the book explores. How it discusses them is not as important as what it discusses. We tune in on themes and concepts and gloss over actual blow-by-blow details. The actual story is for the reader to consume. The review tells that reader whether it will be anything more than just a superficial romp in the park. Creating a full review requires a deep understanding of the work because it transmits that information to interested readers.
Do you see where this is going yet?
When we sit down to create our novel, we should already be able to write our book reports on it. Not just the simple ten-year-old perspective, but the junior-high exploration and our adult book review versions as well. This may sound odd, but it's true - each of these has a place in our writing, and if we can't write them before we write the book, we probably aren't ready to write the book.
In business, these three concepts are usually discussed as: The Elevator Pitch, The Proposal Pitch, and The Presentation. However, I like my version of the ten-year-old perspective, the junior-high exploration, and the adult book reviews. The first one says what happens, the second one expands into ideas, then finally themes and messages are discussed. And whether in business or in writing (and always in the business of writing), they are all necessary to know before anything is done.
If you are working on a major project and find yourself hung up on some part, put these ideas to work. First, write your ten-year-old perspective of the story - one-hundred words to tell the adventure. If you can't package that briefly and succinctly, you are having problems. Second, go to junior high and write your exploration of the story; what will engage the reader besides basic actions, character and dialogue. No more than 250 words should do it. Then write your book review, explaining in about one-thousand words the major interactions, concepts and conflicts, and what should drive someone to pick that book over all others.
If you can't write these, well, that's probably the problem - you don't fully understand the story. If you can write these - and actually do write all three of these - then I guarantee you will resolve what is holding you back.
Of course, if it also inspires you to read Lord of the Flies again, well, that might help too.