- It’s not the same (only more/bigger/badder) problem s/he face before. You can’t just add one or two more gunmen to the shoot-out. It has to be something different, like the house suddenly caving in on them.
- It has to be unexpected.
- It has to flow naturally out of the story so that once the reader sees it, they’ll say—of course that happened, it had to, but it was sure a shock. For example, in a ghost story, your intrepid investigators are attacked by some sort of paranormal entity while trying to help the family who just inherited the house. Blah, Blah. Then suddenly, they are shot at! By another group of competing investigators, who are there because they’ve learned there’s a chest of cursed gold hidden, blah, blah. A physical attack by humans is not what your average ghost story fan expects. It’s not what your hero/heroine expects, either. (The gold isn’t expected by your hero, either, so it’s a two-for-one moment, both information and obstacle.)
New information and obstacles must be present in every chapter—hopefully, something new is delivered in every scene—to create a page-turner. That’s something new happening three times a chapter, at a minimum. That’s the structural requirement to keep a book from having a sagging middle, and once you create the structure, then you can daub on the paint to make it look pretty, too, by shortening sentences, adding more dialog, etc, in those specific high-tension spots where the pacing has to increase to the fastest pace.