If you wander down a hallway and happen upon a fellow librarian pantomiming martial arts moves in his office, it is best not to disturb this person.
Though it may look hilarious, don't laugh. Do your best to sneak away unnoticed and never mention it again. Your coworker may indeed be a ninja armed with throwing stars, a sword, and the ability to rip out your heart with his bare hands. Better to err on the side of caution.