There are a handful of requirements for being considered, biologically speaking, alive. And while I don't have my biology textbook with me, I think the number is seven. (I remember remarking a few years ago that on a given day I'd completed 4 of the 7 requirements, and that this was good enough to win the World Series.)
Suffice to say...Living things grow. Living things consume. Living things eliminate waste. And living things respond to their environment.
For example, if a room is cold, a living person will put on a sweater, turn up the heat, or start moving about in order to warm up. This is normal behavior. If there's a stimulus, you respond to it. If there's a sudden bright light, you blink. You can't help it. That's what living things do. And if you can sense that there are other people walking closely behind you--if you can hear their footsteps, or if you just use common sense, (since you're walking in New York City in the middle of the day)---then that's a stimulus. You're supposed to respond. If you are walking slowly in the middle of a narrow sidewalk, and there are people shuffling slowly behind you, either move decidedly over to one side of the sidewalk or else quicken your pace.
If you don't respond to the presence of people behind you, either you're retarded, of you are not techinally alive, and in either case you ought to be brushed aside like strands of beads suspended in a doorway.