Details.
Either a lot of details of the monster/killer or few details.
However, for some tension building its better to not focus on the thing (Monster/killer) directly. Try describing how the surroundings look or feel, maybe it's raining hard so it's harder to hear anything incoming. Or maybe it's so silent, even taking a breath might be too loud. Or the sound of their (protagonist's) heartbeat can be heard so loud they might think the monster/killer can hear it too.
Any ways that's all I've got for right now!! Let me know what you think makes written horror scary.