on Jane Friedman site:

Imagine you’ve just picked up the latest buzzy thriller, the one that has everyone on social media raving about the “shocking twist.” You start reading, and you’re really into it. It’s a murder mystery with no shortage of suspects—the victim Jane’s cheating husband Hugo, her jealous best friend, her unstable coworker, an obsessive delivery person—and you’re having fun guessing which one is the culprit. You turn to the final page, full of anticipation … only to discover the murderer was the victim’s long-lost twin sister, a character who was never once mentioned in the preceding 300+ pages. What?

Is that a twist? Technically. But it’s also an unearned ending. The author didn’t take the time to lay the groundwork for that conclusion, and the reader is left feeling unsatisfied, even cheated.

Let’s discuss some techniques and craft elements that will help you craft an ending that feels both surprising and satisfying. Note that while I focus on thrillers in this article, earning your ending is important across all genres. No matter whether you’re writing a thriller or a romance or upmarket fiction or something else, you want your reader to feel fulfilled by the ending.