Tension in fiction comes from knowing that something is wrong and waiting to see what will happen. That’s as true of a relationship between characters as it is of Alfred Hitchcock’s bomb under the table. As in Hitchcock’s example, whether this creates tension or a surprise depends upon your point of view.
There’s a scene in Peter Higgins’s book Wolfhound Century that includes two characters – Lom and Maroussia. Lom is a policeman. Maroussia, a dissident, doesn’t know this. The readers do, and we know that she won’t respond well. Throughout the scene, we’re waiting for the information to drop. We’re waiting to see how she reacts. There’s tension here.
Imagine that scene if we didn’t know the truth about Lom. There’s no tension from the relationship now, but there is a surprise coming. A twist has been set up.
False relationships create tension and twists out of the characters we care about. That makes them a particularly powerful source of tension. Think about the film The Truman Show or the TV series Chuck. Most of the drama and the humour in those stories comes from that tension. We know that the relationships between characters are not what they appear. We’re waiting for that to resolve.
How you show this to your readers depends on how you write. But whatever your approach, it’s a powerful tool.