Everything I write in my suspense/thrillers is colored by perception. The events that unfold are told through the lens of a particular character, whether it's the Irish detective Ryan O'Clery, the psychic spy Vicki Boyd, the Irish CIA operative Dylan Maguire, or others. It's their background, their position in the story, and all that makes them who they are, that slants the action in one way or another.
To see just how critical perception is, just look at the world through an animal's eyes. I have three dogs - a collie, a Jack Russell, and a Jack Russell mix. Every day someone pulls in front of my house and stops for anywhere from a few seconds to a minute. Each time, the dogs rush through the doggie doors, descend together in front of the gate at the end of the driveway and bark their heads off. And each time they frighten the intruder away. Yet he's back at the same time the next day - except he takes Sundays off.
To my dogs, they have protected my house and its inhabitants. They have pulled together, ready to fight, ready to warn everyone within hearing distance of the stranger at the edge of our property. They remain ever diligent for more intruders, ready to repeat the performance whenever another threat is detected.
Who is this stranger, who pulls in front of my home? Who stops his car and then pulls away after a few seconds - or up to a full minute - later?
It's the mailman.
In a book, it is the unknown that increases the suspense. It is seeing things through a character's eyes when the character has not yet figured things out. Of course, for it to be a good read, a satisfying read, at the end the stranger can't be a mailman - unless, like Three Days of the Condor, he comes bearing weapons with the intent to kill.
Sometimes, it's all in perception.