When I first faced fear as a child/man my instinct was to fight hard and fast. I have several brothers and it may be that I had to learn this reponse from the cradle in order to get a second pork chop.
It was in Viet Nam that I first encounterd the flight response - incoming mortars... and the need to run is amazing. It was there that I learned the response that Chris refers to - to be calm and evaluate, clearly not the same as freezing. The only time I have ever frozen was when my first wife told me we were getting a divorce.
These are human responses and training in Aikido gives us the option to overcome instinct and transcend the normal human condition. After 20 or 30 years and thousands of punches aimed at your head you should be able to calmly evaluate a threat instantly.