I know we'd all like to think of ourselves as the situational awareness ninjas who always notice everything...
But, of course, the fact of the matter is we are all human and we all make mistakes. We are assuming our "humanity" in this scenario and that you have failed (for whatever reason) to check your back seat before entering your vehicle and have noticed nothing amiss upon entering the vehicle (such as signs of forced entry, etc).
So, let's say you get in your car.. doesn't matter if it's at home or the grocery store or wherever.
Let's say you are parked facing and against a building or other larger object/vehicle (your home, garage, a bank, the store, SUV, truck, tree, etc) meaning you cannot drive forward without running into said object.
You have put your seat belt on, started the vehicle and even put it into reverse and look up and into the rear-view mirror to back up and see a stranger. He (or she (doesn't matter)) has not made any demands or said anything yet. You cannot see if he has a weapon nor has he verbally threatened you.
What do you do?
PS... and for those of you who do not have back seats because you drive a back-seat-less vehivle or claim no one could fit in your back seat for whatever reason, just play along by assuming you are driving a vehicle that is not only equipped with a back seat but one in which a someone could sit.