The mind seeks patterns. Familiar shapes and known order are comforting, they help make sense of surroundings. When he started seeing faces in odd places, Callan thought he was simply quite adept at finding patterns and went about his day.
When the faces started talking, he thought he had gone mad.
When others heard the voices too, he learned he had a problem.
Bakery, cinema, pub, office; he saw faces and all present heard voices, but only when he occupied the space. Where he was the faces could speak. Could be heard.
Callan was fine until the faces grew bodies.