In connection with yesterday's first page tip, use those first pages to firmly set the type of writing you intend your book to be.
I recently read a novel by an author I usually like. It was advertised as a suspense thriller, which I love. Chapter one started with a bit of a thrill in the form of a flashback but then it goes straight into romance mode. For eight chapters. I felt a bit cheated. It was chapter 9 before the antagonist and "thriller" part of the conflict was introduced. Somewhere around chapter five, I seriously re-read the backliner to make sure this was really going to be a suspense novel—eventually.
IMHO, that's just too long to wait before setting up the current conflict in a suspense novel. If I hadn't read and liked this author before, I'd have given up and quit reading long before chapter 9. If I'd wanted a romance, I would have bought a romance, but I bought a thriller, I wanted a thriller—and wading through nearly 90 pages of romance was ticking me off.
Now, I'm not saying you can't have romance in your thrillers. You certainly can. While not absolutely critical, mixing a developing romance in with your murder adds interest to the plot and gives the reader a little time to breathe. But if you're writing a thriller, you need to get to that story right away. It needs to be the main plot line—starting with chapter 1. Diverge to romance every other chapter or part of the chapter, but keep us on track with the suspense.
If you need some backstory and character development, that's fine but build the tension a little more in every chapter to keep us hooked and always remembering that this is a suspense novel. You don't want to lose your readers who thought they were getting one thing, but get so much of the other right at first that they never get to the good part.