Every writer learns early on the importance of the first fifty pages in a novel (or the first paragraph in a short story). Writing teachers and professional writers drum this into the student in every class, and add the comment to every manuscript they critique. And there's nothing wrong with this advice. The opening of any work of fiction is crucial to establishing the story and then the author as a worthwhile storyteller. But there is a downside to this advice.
For the decades I've been reading fiction in all genres, and especially mysteries, I've often been hooked by the opening paragraphs and then watched the story fade. This is more likely to happen in literary fiction than in crime fiction, but it is a problem in every genre. Sometimes this is called the problem of the sagging middle, or the ending that is more "talky" that anything else.
The emphasis on the opening pages or paragraphs stems from a very practical consideration. Editors read with the hope of finding something that will tell them the book (or story) isn't working and they can stop reading this one and move on to the next in the pile of mss filling their offices. The emphasis on the first fifty pages is basically a survival tool for editors. There is an assumption that if the writer can get the reader fifty pages into the story, he or she will want to keep reading to find out what happens. That isn't always true, but the belief is strong. I've fallen into the trap set by this dictum of the first fifty pages on both sides.
As an editor for The Larcom Review and The Larcom Press, and an occasional reader for contests, I looked for a sign that the author couldn't sustain the story over three hundred pages. And I looked for that sign in the first fifty. If a ms seemed promising I skipped ahead to page one hundred and then two hundred, to see if the writer could still keep my interest.
As a writer, I have found myself going over and over the first few chapters, to make sure they set the stage, establish character, and pose an enticing problem. But I know there is more. I have to avoid the trap of lavishing attention on the opening and skimping on the rest of the book.
To make sure I don't fall into the trap of focusing more attention on the beginning than the rest of the book I work on the ms in chunks, with a list of clues/details that have to be distributed throughout the story.
Despite my best efforts to avoid the trap of the first fifty pages, I fall into it just like every other writer. And then I work to climb out by giving as much attention to the rest of the book.