Dramatic irony

Dramatic irony is the state in which the reader or audience knows something a character does not, so that every ordinary line the character speaks carries a second meaning.

The comparison that makes the case comes from the screenwriting site Scriptshadow: a sudden twist — the heroine's boyfriend is the killer — hits an audience for about five seconds. Dramatic irony, where the audience learns he is the killer and she does not, can hold that audience for half a screenplay.

That is the whole trade. A withheld secret buys one moment of surprise. A shared secret buys every scene until the character catches up, because the reader is now working on your behalf: watching her pour him a drink, hearing an innocent question as a threat, willing her to look at the thing on the table.

It is also the cheapest subtext available. No line has to be rewritten to carry a double meaning — the reader's knowledge supplies it.

The cost is patience. Dramatic irony only pays while the character stays ignorant, and a reader who has been ahead of the protagonist for too long stops feeling tension and starts feeling impatient. Decide in advance which scene closes the gap, and make the closing a disaster rather than a conversation.