More than that is the mental game. You have to be ready for anything, whether it’s something as simple as someone entering or exiting (anticipating movements at all times) while assessing everything around you for possible threats. And 99.9% of the time, you’re just standing there, motionless, with absolutely nothing to keep your mind occupied.
It would be like being in solitary confinement for half your day every day, but you don’t get to move. Just stand there, perfectly, and move only exactly when needed, how you’re needed, and oh, be ready to die if necessary to defend people you probably don’t even like.
The mental discipline is incredible. How do you keep your mind from wandering? How do you stay on mental high alert for so long?