For me the line between mystery and suspense lives in the verbs — what you do with that intrigue. Mystery leans into words like 'enigma', 'puzzle', 'riddle', or 'conundrum' because the reader's job is to solve; the narrative hands you clues and waits for you to piece them together. I use 'enigma' when I want a slow-brewing intellectual draw, the kind you get in 'Sherlock Holmes' pastiches or an old-school whodunit where every line matters. 'Puzzle' and 'riddle' are great when the structure itself is the attraction: think locked-room stories or game-like narratives that invite participation.
Suspense, on the other hand, benefits from synonyms that carry motion and heat: 'tension', 'dread', 'uncertainty', or 'foreboding'. These words push the reader forward rather than backwards toward a solution. When I describe a thriller to friends I might call it a 'conspiracy' or a 'manhunt' because those imply stakes and momentum — there’s danger, decisions, and a clock. Films like 'Jaws' or 'Rear Window' (and books that replicate that feeling) are all about sensory anxiety, so 'dread' fits better than 'mystery' there.
When I pick a synonym for blurbs or tagging, I match the reader's expected posture. If I want them solving, I use 'enigma' or 'mystery'; if I want them clenching their jaw, I use 'tension' or 'dread'. Sometimes both live in the same story, and then I reach for hybrids: 'intrigue' for atmosphere, 'puzzle-driven tension' for pacing. That blending is delicious and keeps me coming back to stories that do both well — I always feel sharper after a good mix of brain and pulse.