Secondary characters who aren't just props. In weak suspense, everyone exists solely to be a suspect, a victim, or a source of exposition. In great suspense, the supporting cast has their own lives, motives, and ambiguities. They deepen the protagonist's world and make the central mystery more complex. Is that friendly coworker genuinely helpful, or subtly manipulative? Does the nosy neighbor have a sinister agenda, or are they just lonely? When every character feels real and capable of anything, the field of possibility widens, and the suspense multiplies.