In this project, the phrase names the gap between a reassuring impression and evidence of harm. Jack presents himself professionally during the interview; Wendy’s account in No.7 reveals a different side of him. Ullman similarly describes Grady as a normal individual before recounting the murders.
The distorted inference is that appearing ordinary makes someone incapable of violence. This is an observation about the film’s framing and the viewer’s assumptions, not a clinical diagnosis of a character or viewer.