This is an interesting subject because it is so poorly studied. Extreme calmness could also be called hypo-neuroticism. Hyper-neuroticism is extremely well studied because there are performance consequences to high anxiety and physiological consequences to extreme anxiety.

The most extreme lowest of neuroticism is generally guessed at lesions in the brain that prevent the formation of fear memories from the amygdala. Lesser extreme low neuroticism is possible from conditioning due to repeated participation in high stress situations. This is typically confined to emergency medical technicians/doctors, deployed military, and certain law enforcement officials. For example, combat medics are 4 times more likely to develop PTSD from a combat encounter than their infantry peers, because the stresses are equal but the impact from performance failure is not equal.

Low fear personalities have fewer restrictions on their decision making capacity. That loss of inhibition can be both good and bad. This is why ethics are so profoundly and critically important to the culture of the US military because a culture of rules serves to both replace the natural limits of chaotic actions and simultaneously serve as a framework to enable more predictable output.