Only defending when not at the center of a province

Is it possible for a unit that is stopped on the middle of a road in a province which is not the center point to only defend when he is attacked by a another unit? I know that units only defend at the center and will only counter attack if you want them to. But if they are sitting idle on a road and are attacked, I think that they automatically counter attack after the unit attacks them first. Any insight would be helpful!