As I explain to my wife, when I am off I can walk or I can talk but I can't do both.
In a tight space or crowded room I get an overstimulated sensation very similar to that I get when on and dealing with a traffic crossing as a pedestrian. Multiple input streams leading to freezing.
Interestingly enough, one of the ways they assess the effects of prenatal insults to rats is how they react to being in an alien environment with one of the critical measures being "freezing".
I'm never bored.