We are all detectives.
When I meet a new person, I immediately begin gathering clues about them. The process starts with a first impression and continues as long as we interact. I watch their actions, listen to their words, judge their reactions, and try to piece together the hidden inner world of that person. I get to know them.
The longer I know a person, the better I can interpret body language clues and guess their inner state. Sometimes I may even be able to predict their reactions. But no matter how well I know a person, there will always be surprises and more mysterious clues to decipher. No one acts reasonably all the time, and no one perfectly fits into a mold. Sometimes there may be no answer, but we still want to understand.
Every human being has hidden complexity in their inner world, something their friends can only guess at through interpreting secondary clues. When you write a story, make it more realistic by imitating this hidden complexity. Drop clues that indicate there’s more to uncover, even (and especially) if there’s no clear interpretation of these clues. Humans are meaning-makers, and we’ll still try to understand. Your readers will try to solve the unsolvable, just as each of us does in real life.
Hint at secrets. Why did he frown when she asked about his girlfriend? Why does she hate the smell of coffee? Why won’t he speak about his past?
We are all detectives, and we want to know more.