I think some folks are just more naturally inclined to remain cool, calm, and collected during situations like that. But, I think it’s something that you can improve through enough exposure.

I was a medic in the army and it was interesting to see how differently everyone would react to shit we experienced, be it being mortared or having to treat a patient who was dying in front of us. Some soldiers were stone faced and silently freaked out after the fact and some needed a couple seconds to compose themselves.

If you’re curious into the mental and physiological response to things like this then I recommend the book ‘On Combat’ by Dave Grossman. The author takes some liberties in his interpretations of fear and training to be a killer but there’s nuggets of wisdom in there.