First point, moving off the X refers to the hit sight, usually meaning they planned to take you out at that point (usually a choke point for some reason). So they have the drop on you, they are drawn and either shooting or going to be very shortly. Most people are not going to be in this situation unless it is combat.
Everyone here has very good points. But remember it all comes down to situation. I really don't know a single civilian or military person that won't have some sort of movement when a gun is fired, especially if you realized that gun that fired was at you. I don't know how many people have ever been shot at or how many have ever had to pull and shoot at someone. But I know from experience everyone moves at the sight of a gun. I will state if you aren't moving you probably won't live through it. Most times the bad guy has the drop on you. Move, pull/evaluate, engage/run. This is the most common thing I have ever been trained no matter where I have been.