React! Do something! Anything!
These words appear in my training all the time. They appear in my blog from time to time. I will argue that it is far more important to simply react to a spontaneous attack than to have a choreographed response. It takes years of study to smoothly execute specific responses. Obviously, our end goal is to reach a level where the specific responses occur spontaneously, but that takes time.
Freezing up during a real attack is a very real and common occurrence. Most people simply aren't equipped to handle what is happening. I don't mean that they don't have the physical ability to deal with it, I mean that their minds have difficulty accepting what is happening. It is so far out of the ordinary that the brain can't compute what's happening. So they freeze. There is lots of footage on the Internet showing people just standing there while they're being assaulted.
To oversimplify, during this freeze, the brain is searching for some frame of reference from which it can formulate a response. Past experience is often where it finds these learned responses. The faster your brain can access this frame of reference, the faster you respond.
Most adults are not used to being assaulted. They freeze because they can't find any reference from which to draw a response. (this differs from pure survival instinct, which is a topic for another time).
Luckily, the brain is fairly flexible. It can often be satisfied with locating similar experiences from which to draw a response.
This is why we train. We are building up these pools of experience to draw from. The more we have tucked away, the faster we react.
This is why it's so important to train with a serious mind, utilizing visualization techniques and concentrating on each technique. This way, when a real attack comes, the brain doesn't need to search to find a response.
This is why you should never get frustrated when you go to the same technique a few times under spontaneous randori. Yeah, you may have wanted to showcase five techniques, but it's far better to only do one or two, but do them immediately.
Far too many students stop mid defense when an attack is thrown because they didn't do what was asked from them.
Again, it matters less what you do, as long as you do something. Anything. React.