What makes people tense? Lots of things. But what makes them most tense is when they’re unsure about something. For example, they think they might know that their boyfriend or girlfriend or husband or wife is having an affair, but they aren’t sure. Once they’re sure, one way or another, they might feel a lot of things but the tension is different. I could be wrong about this but I think the peak of the tension is not knowing.
When your goal is to intensify tension one of the best ways to do that is to put your character in this place of not-knowing and carefully describe what he or she goes through when they’re there.