Term for: People acting normally while others disbelieve
Juxtaposition
NOUN
The fact of two things being seen or placed close together with contrasting effect.
This word describes exactly the sort of scene you gave. I am not sure if you want the word to be used in the book text or not, but if so, something like this might work: "The calm reaction of Dr. Reynolds at seeing Boo Radley juxtaposed with Scout's reaction highlights her utter disbelief of what she is seeing."