Fear of the unknown is probably one of the most common fears present in humans (maybe all humans?). For example, humans fear death because they don’t know what will happen after (unless you have strong faith), or the dark, because they can’t see if there is something or someone there.
Despite this, humans are also drawn towards the unknown. They do their best to find everything they don’t know. Whether it is to activate adrenaline associated with fear or to curb the fear with knowledge, I don’t know. Regardless, it’s important.
From this fear comes suspense. Suspense is the anxiety that comes from belief of the imminent revelation of an unknown outcome. For some reason (maybe our brains’ need for closure), we want to know what happens. What the outcome will be. If we take away the unknown, is the suspense defunct? Somewhat, but not really. As to why, my guess would be that our brains interpret the outcome as an unsure outcome every time around. Maybe we subconsciously believe that a different outcome is somehow possible.
Anyway, there are two main types of suspense that appear in fiction (that I can remember at time of print):
The suspense that can be described by the following scenario. “Alfred Hitchcock was asked to define suspense. He told the interviewer to imagine two people sitting at a table at a café. Under the table is a bag. In the bag is a bomb. The characters don’t know that the bomb is there but the viewers do. That, he said, is suspense.” (via Crime Fiction Collective)
The other type is one where the reader knows as much as the character. In the bomb scenario, the character might have gotten a threat and nervously looks around while talking to a man who he shouldn’t have been talking to.
If I can paraphrase David Baboulene here, it’s all about subtext. To create suspense, someone has to know more than someone else. Either the reader and the villain knows there’s a bomb and the hero doesn’t, or the villain knows there’s a bomb and the reader and hero don’t.
However, the most important part of suspense (or tension) is that the reader must care about the outcome. Humans want to know the unknown, but there are so many unknowns that they prioritise them. If they don’t care about your characters, they won’t care about the outcome, and the book, to paraphrase James Scott Bell, will be left behind unread in the train.