Today we're going to talk about how to create suspense, but first let's just defuse this bomb...
I've talked in an previous quick guide about reader/audience surrogates, and how they are used to create an effect on the reader, such as elation or fear, by having the surrogate experience that emotion. An occasional exception to this, however, is suspense, which can be created by having the reader surrogate act oblivious to the situation.
So, what creates suspense?
Alfred Hitchcock had a famous example that is worth mentioning here: you have a room full of people acting as though nothing is wrong. What the audience knows - and the characters don't - is that there is a time bomb underneath one of the tables. The longer the scene goes on and the bomb doesn't explode, the greater the suspense.
This is a case where the suspense is created by the disconnect between what the audience knows and what the characters know. However, suspense does not rely on that knowledge gap. The scene can still be suspenseful if the characters in the room know about the bomb and are trying to defuse it.
Breaking this down a bit, we can say that the creation of suspense relies on two main factors:
The existence of a situation in which a meaningful consequence will happen to the characters in the story.
The audience/reader knowing of and understanding the importance of said consequence.
The consequences do not need to be negative for suspense to exist - one character proposing marriage to another in a romance creates suspense through a situation where the end result will be positive for both characters involved - but in most cases a negative consequence is what is used. Very often, the consequence involves the character in question coming to some sort of physical harm.
This leads to the question of character awareness - is it better for a character to be aware or unaware of the situation for the purposes of suspense? This depends entirely on the story being told, as well as what creates a greater level of jeopardy for that character. A character going shopping while being hunted by a sniper is in greater danger if they are unaware of the threat, as that prevents them from taking actions that could lead to their escape. Likewise, if a character is part of an emergency response team, having the threat appear in the form of a bomb that they must defuse creates far more danger than if they were a civilian in the same situation - the civilian would be evacuated, while the ERT member must sit beside the bomb and take actions that could make it explode prematurely.
Regardless of whether they are positive or negative, it is very important that the consequences be real and credible. Suspense cannot exist if the reader doesn't believe that the author would follow through. This is one of the reasons why "save the world" plots tend to falter in this respect - the stakes are so high that deep down inside, the reader or audience knows that the world isn't going to be destroyed. This doesn't mean that a story can't create suspense using a fake-out (the bomb threat being a hoax, etc.), but it does mean that once the fake-out has happened, that type of threat will no longer be able to create the level of suspense it once did.
Credibility is often best established through precedent. For example, if in the above-mentioned love story the consequence is the two characters getting engaged, the story needs to establish that they have fallen in love. If the story is about a sniper hunting shoppers, we should either see the sniper practicing with perfect aim (establishing that s/he will hit whatever s/he is aiming for) or shooting somebody else before their potential victim goes out on their shopping trip. If the consequence is a bomb going off, we should either see the antagonist display skill at bomb making or a bomb exploding elsewhere.
Once the consequences are credible, suspense then relies on proximity to said consequences taking place. The closer the story is to the consequences happening (the timer on the bomb counting down to zero, etc.), the greater the suspense will become. This will last up to the point where the consequences either take place or are averted, at which time the suspense dissipates. It is important that the reader understand that the proximity to the consequence is increasing - the audience or reader needs to see any escalation (the bomb timer ticking down, ruffians in a bar pulling out their knives, etc.).
To sum up: suspense relies on the existence of a situation in which a credible consequence will occur, and the audience or reader knowing and understanding what will happen. So long as these two elements are in place, your story will be suspenseful to the reader.