I think we have to remember that people who aren't psychotic (or neurotic) can only see or perceive the sufferer from their own point of view. They can only observe the outward signs, they can't see the feelings of the sufferer.
In schizophrenia there is a detachment from reality, and the sufferer doesn't know the difference between what's real and what isn't, or they might be aware of what's not real but they can't overcome the feeling that it's real. They might then act in a way which reflects the delusions they're under. That might mean being violent or agitated, or it might mean curling up in a chair and appearing to be devoid of any emotion.