Imagining likely scenarios and thinking out practical solutions to those threats will go a long way toward overcoming the fear and panic that accompanies a deadly criminal attack. Without this prior planning and study, a person is very likely to react to surprise and fear by drawing a gun—and even using deadly force—when it is simply not necessary.
One of the ways to overcome this problem is by establishing what Col. Jeff Cooper called "the mental trigger." Through our thought, research and study, we determine those actions of another that are a clear indication hostilities have been opened. Essentially, we determine, "If he does this then I must be prepared to stop him." Until this mental trigger is tripped, our guns stay holstered. Even after the mental trigger is tripped, the armed citizen may not fire, but he is ready to do so if need be.
Suppose you are reading in your den and, because you are in Condition Yellow, you are immediately aware of a sound at the back door. Going to the back door and looking out a window, you see someone standing on your back porch (Condition Orange). Now you see the man produce a small pry bar and begin working on the back door (Condition Red) and your mental trigger is tripped. You should now be prepared to do battle if necessary. Your mental trigger would be, "If he enters my house with that pry bar (a dangerous weapon) I may have to stop him."
At the point the man has produced the pry bar and gone to work on the back door, you should have drawn your handgun. It is at this point that you know for certain it is not a next-door neighbor who is about to knock on the door. At this point, it becomes clear that the individual is engaged in a criminal activity that could escalate to include a violent criminal attack.
It should be made clear the mental trigger is not your decision to shoot. The episode may be resolved without having to fire a shot. But the determination of whether or not a shot is going to be fired is entirely up to the bad guy and how accurately he evaluates the mess he's gotten himself into.
The study and evaluation of likely scenarios gives us the ability to overcome fear and panic. It helps us develop a plan. As Col. Cooper often pointed out, one should be able to say, "They told me this could happen and I know just what to do about it."