I can see this in some of the people I have trained with at my club. It is a subtle 'hardness' in their character. They can be very nice people but they are under no illusion about what would be required in a life-threatening situation.
It's a personality thing, it comes down to how a person views themself. About once a year I find myself in some kind of potentially violent situation because I've seen something I don't like the look of. For instance one night I was walking home from the pub and I could hear a woman screaming. I went and had a look and there was a guy dragging a woman along the ground kicking and screaming into a house, so I stepped in.
I've come to the conclusion that I do it because I believe that that's who I am. I see myself as the one who's busy dealing with the problem when everyone else has frozen. I see myself as the one that takes charge of a situation and is cool and calm when everyone is panicing. I'm the lunatic that will walk into a situation because it's the right thing to do, even if it is nigh on suicidal to do so. I mentally reherse this.
I think it's in hagakure where it says, "Meditation on inevitable death should be performed daily. Every day, when one's body and mind are at peace, one should meditate upon being ripped apart by arrows, rifles, spears, and swords, being carried away by surging waves, being thrown into the midst of a great fire, being struck by lightning, being shaken to death by a great earthquake, falling from thousand-foot cliffs, dying of disease or committing seppuku at the death of one's master. And every day, without fail, one should consider himself as dead."
This is something I take seriously. In my experience the people that don't freeze are the ones that give serious thought beforehand to how they're going to react in a given situation and who have a strong sense of who they are and how they react when in trouble.
I suppose this is why we have war chronicals and books like the Illiad so one generation of warriors can say to the younger generation, "This is who you are, this is what you will do in a critical situation, this is how you will conduct yourself." It forsters a sense of identity that they could fall back on when they found themself in the dodo.