The author knows how to build suspense. We're put into the same position as Adachi. We have no idea what Shimamura is thinking, but we have the instinct that something isn't quite right.
I've been in a position similiar to Adachi. Reading this is giving me a similiar pain in my stomach. Good art draws emotion from the viewer. I am feeling physical discomfort from how well this captures the dysfunctions of two people combining to create a true Hell