Working in an elder care facility, I have several residents who have schizophrenia and experience delusional behavior. They are of course medicated and their delusions are minimized. One of my rules is, never tell a person what they are seeing is not there, because who are we to tell them it's not.
This movie was a little confusing at first, but there are clues right from the beginning that this man has significant mental health issues. It gave good insight into what a person might experience when they experience delusions. Very well made. It's not often that movies are made to force you to pay attention and think.