One of the most common defenses is to avoid whatever you fear. Another response, in some situations, is to freeze or become paralyzed. Yet another response is to become angry. Some people cry. Or, you can use fear to become mobilized.
Fear also generates internal changes, often described as anxiety.
These changes involve blood flow, breathing, skin temperature and muscle tension. Whatever your responses the experience of fear generates behavior. I would strongly suggest that a person who has no fear is a disaster waiting to happen. Once when I was still doing regular boxing, a stranger showed up who hadn't fought in several years. He wanted to spar as he considered resuming training.