As implied by the idiom, the Reality Check is an intrusion of reality upon the delusions the character brought into his new situation. Here, the character and the audience finally realize he is waaaay out of his depth, and the situation he’s stepped into is far more severe than he initially concluded. The stakes inherent to achieving his Transitional Goal become clear to him for the first time, and whatever misconceptions he had regarding the Unfamiliar Situation are laid to rest as the new world reveals its inherent dangers.
