it's not just about non-stop action in an action story or horror in a horror story
Two points.
1. Novels must have different content, because otherwise they'd be unbearable. But if it's a genre, say, action short story, is must be mostly that, or it isn't an action short story. If it's mostly a character drama, then it's a character drama, regardless of whether they shoot in the end or no. If they make love, it still isn't erotica. It's a character drama or a slice of life.
2. Build-up can not be longer than genre content itself. I maintain that if you have more build-up than content, you're wasting the reader's time. Look at any Stephen King novel with supernatural elements. That guy can be slow. And even in his work, at which percent of the story does the horror strike, say, in the Shining? Somewhere around 10% in? And the supernatural - telepathy - appears almost immediately. If it was a short story, it would have been the middle of the first page.