That implicit laugh at 1:39
Psychologist Dr. Karen Fletcher has a session with Barry about the incident at work in order to bring Dennis' personality forward and talk about Kevin's safety. Dennis claims that Kevin is too weak, and can't realize how powerful they can be.
Dr. Karen doesn't believe that The Beast is his 24th identity and persuades Dennis, that it's just a fantasy.

Clip from the movie Split, starring James McAvoy. His acting is
overwhelming and leaves the audience completely speechless.