terror is

(Marcel Duchamp, Nude Descending a Staircase, 1912)

Watching him descend the stairs by himself.

He stands on the top step facing the banister and holding the spindles
with both hands.

He kneels to his knees and lowers his left foot down to the next step.

He readjusts his weight, moves his hands over to the next spindles, and
brings down the trailing foot.

He repeats the above until he's reached the landing.

While I remained less than an inch away the whole time, I didn't touch
him.