I've always found the best mysteries start with a character encountering someone who seems totally insignificant—a shopkeeper who's a little too chatty, a librarian who remembers a detail others forgot. That's where the secrets hide, not in locked rooms or coded letters. The enigmatic stranger isn't necessarily the villain; they might be a victim themselves, or a piece of a much larger puzzle. Their story always contains some small thread that unravels everything else.
It's the quietness of these moments that gets me. A fleeting expression, a hesitation before answering a question—these micro-interactions hold more weight than a whole police file. The detective, or whoever our protagonist is, has to be the kind of person who notices that flicker of recognition, who wonders why the old groundskeeper stopped talking mid-sentence. The secret isn't revealed because someone confesses; it's revealed because the right person finally saw the truth in a stranger's eyes.