Try it yourself. Go out, pick somebody, and watch them. Take your phone and a notebook. Persist. What begins as a confluence of yours and another person's journeys, on the train maybe or leaving a cinema, gets into an entanglement. You follow, feeling that it's not really following because you're going the same way; then when they at last reach their office you feel the clutch of a goodbye. It's normal. But how many times do you think the person being followed has been you?