Tension is an important part of any story. You want the problems gripping your characters to also grip your readers. But tension is not a one off thing that you can create and leave to do its job.
If tension remains at a steady state it decreases over time. If a guy is in a locked room waiting for the killer to come back and finish the job, and he waits, and waits, then he’s eventually going to stop freaking out. He might even get a little bored.
You either have to face the problem (leading to some kind of resolution) or escalate the tension in some way. But even then not all tension is created equal.
If an asteroid is going to hit the earth in a year, then that certainly needs to be addressed. People will run around preparing to brief the President and so on. If the next chapter is one month later, and then the next one another month and so on... then while that can work in terms of story structure, it won’t be a very effective way to maintain tension.
On the other hand if the asteroid is going to hit in three days, then two, then one, even though that is a similar structure it is going to be much easier to relay a sense of threat to the reader.
Urgency is just one method. Increasing stakes is another.
In most cases how the character is reacting will be a good indicator of how the reader is going to be feeling. And the easiest way to keep the character busy is with new and more terrible problems. This kind of escalation can easily get out of hand.
If the asteroid is going to hit in a year, then at six months Russia invades Alaska, then with three months to go a zombie epidemic breaks out, then while tension will certainly be running high, it could also be seen as getting quite silly.
Not that silly won’t sell. Most blockbuster thrillers have a great big dose of silly that will defy all common sense, but there are a lot of people who like that sort of silly, and once you get caught up in it you can quite easily leave disbelief (and your brain) at the door.
These are all methods that can work, but another way to keep the level of tension nice and high is to double dip.
If Man A holds a gun to Man B’s head and asks him where the money is, then there is tension there. Man B manages to assure Man A he has no idea what happened to the money and Man A lowers the gun, and the tension drops. But then Man B says the wrong thing and the gun comes up again. What happens to the tension?
Not only does it return, but it comes back stronger as the surge of adrenalin shoots back up. The dip in tension allows for a push into a higher level of tension without having to bring in a new set of problems or characters.
This double dipping isn’t just for thrillers, it will work for any scene that involves conflict. What happens in a lot cases, though, is that the tension is raised, then dropped, and then the scene or chapter ends; because it feels like a good place for an pause.
This isn’t a good place to end a scene.
Far better for everyone to think the asteroid has been diverted from its course and then have someone point at the radar and say, “Hey guys, what’s that second blip?”
Letting the reader see a little light through the door before slamming it their face can be as effective as piling on more and more problems, and it is often much more sustainable.