Has anybody ever caught themselves thinking of people around as them as merely being part of the scenery? As if they're not really other humans that you can talk to and get responses from, but rather just there for atmosphere or as elements of a complicated backdrop? Non-interactive and following set patterns they move about like characters in those computer games where you can only talk to certain people, and the rest are purely graphical objects meant as eye-candy.
I sometimes wonder if my hand would pass right through them if I were to try to reach out and touch someone, or if they would react at all if I suddenly started screaming or jumping up and down. Maybe some of the more advanced characters are capable of basic pre-defined responses to questions regarding simple matters, such as how to find a certain place or what time it is, but overall they're only there for effect and shouldn't be touched. I suppose that's one of the reasons people are often startled when accosted by strangers, it's almost as if a rock or a tree suddenly woke up and started speaking to you.
Maybe this is especially true for Japan, where it at times seems like people are going to great lengths to ignore and block out their surroundings, in particular anything that might cause embarrassment or inconvenience. Thus, for example, you'll often see passengers on the train squirm due to having their shoulder used as a pillow by the person sleeping next to them, but rarely will you see anyone opening their mouth, asking the slumbering party to wake up or at least giving them a jolt, since this would cause a scene/disturbance.
In fact, usually the person asleep will continue sleeping until the person being used as a pillow reaches his/her destination and gets off the train. Actually, I just looked up "slumber" on the internet. Apperently it means "a natural and periodic state of rest during which consciousness of the world is suspended". It makes me wonder which of the two persons in the example above it applies to.