When the Dementor appeared outside the compartment door, not only Percy's face froze, but the expressions of all the other prefects in the compartment froze as well. This was their first time facing a Dementor directly, facing such an evil and dark creature.
Regardless of what they had been thinking previously, they now only felt a chill rising from their spines, rapidly spreading through their entire bodies, draining them of any capacity for happy emotions.
"Black is not in here!"
Chapter 617