Defensive driving is all about seeing potentially dangerous situations in enough time that action can be taken to avoid them.
Defensive drivers know that many of the potential dangers can be corrected before the vehicle starts to move, and if not corrected then, can be difficult or impossible to correct later.
Pilots perform a walk around check of their planes before every flight, even if they have just landed to drop someone off. Defensive driving involves a walk around check of your vehicle as well. If a vehicle has been stopped then there has been time for people or other vehicles to move close. This is a potential dangerous situation which defensive driving techniques protect against.
A possible walk around checklist for a defensive driver would start by sitting in the vehicle and checking the mirror adjustment, horn and, if there is one, the backing warning signal. Then the lights would be turned on and a visual check made that they are all functioning correctly, followed by brake lights and turning indicators. During the walk around the defensive driver would also check that the tires are inflated, and the final defensive driving checklist item is to be sure that there are no obstructions, people or animals nearby, especially in any blind spots.
As soon as the defensive driver is satisfied that everything is in order, the vehicle is started and moves off slowly but without delay, so there is as little time as possible for anything to approach the vehicle unnoticed.
Vehicle safety is not an accident! Take the time to learn defensive driving techniques and practise them whenever you are driving, and get a check ride with a defensive driving instructor from time to time.