Man, I get nerdy about this — cover design for thrillers is basically the first heartbeat of a book. Dark palettes with one arresting accent color still rule: blacks, deep blues, charcoals punctuated by a slash of blood red, neon orange, or icy white. One style that keeps selling is the minimalist, typographic cover: big, bold sans-serifs, distressed or layered, often with a tiny symbolic image (a key, a scar, a blurred face). They read cleanly at thumbnail size and scream genre without shouting.
Another hot style is photographic realism: a close-up of an eye, a hand, or a rainy street. Faces sell emotion, but only if the crop is mysterious rather than literal; the less the photo explains, the more the reader fills in the blanks. Then there are symbolic illustrated covers — a single object rendered with texture and grime — which work great for psychological or literary thrillers.
Don’t forget format tricks: matte finish with spot-gloss or foil for hardcover, heavy grain paper for paperbacks, and typography optimized for tiny thumbnails on Amazon or Kobo. If I had to pick a move for an author launching a thriller, I’d pick a bold, readable thumbnail-first design and then add tactile premium touches for print. That combo still gets me excited when I walk into a shop.