Show them once so they know

 This is a great tool for storytelling. It is almost always invisible to an audience.

In the film The African Queen, there is a sequence in which their small boat is trapped on a sandbar. Humphrey Bogart's character must get into the river and try to pull the boat free by hand. Unable to free the boat, he climbs back aboard. When Katharine Hepburn notices that Bogart has leeches on him, Bogart goes into a panic. He is deathly afraid of, and disgusted by, leeches, and he trembles in horror. He is truly shaken by this event.

Shortly after the leeches have been removed, the characters realize there is nothing they can do to free the boat by staying aboard. So Bogart must try again to free it by hand. It means he must get back into the river. You can almost feel his dread as this realization sinks in.

When he starts down into the river we know how brave he is. We know that he's facing an obstacle that is particularly large for him. It is almost like he is his own clone character. We can measure his bravery next to his fear seen before.

This kind of invisible ink can be used a couple of ways.

Close Encounters of the Third Kind is a film that makes use of UFOs as part of its reality. Here is a famous scene from that film.

Richard Dreyfuss is in his truck at night and he is lost. He stops his car in the middle of the road to check his map. Behind him, we see a pair of headlights drive up. Dreyfuss waves the car around. The driver goes around Dreyfuss's truck.

Very shortly after, the scene is repeated almost exactly. Dreyfuss is stopped and looking at his map when a pair of headlights drives up. Without looking up from his map, Dreyfuss waves the car around. Unbeknownst to him the lights behind the truck rise vertically. (Good use of superior position, by the way.) It's a creepy scene.

It works so well because we saw the previous headlights behave in a normal fashion, so now we have a comparison for what is normal and what is strange. Very smart storytelling.