There's no such thing as gradual onset schizophrenia all cases are acute onset. Also you don't get schizophrenia from depression. In severe depression you can develop psychotic symptoms such as auditory hallucinations which has overlap with schizophrenic symproms.
Schizophrenia is a condition which involves a number of psychotic features. Psychotic meaning losing sense of reality. When dealing with a person with schizophrenia you need to understand that this isn't there fault, they can't control it and it could happen to anyone. You've got to understand that the symptoms they perceive are as real to them as anything you see or hear or believe. It's not something you can convince them isn't real using logic. Don't overly challenge them, but most importantly have sympathy and remember this could happen to anyone, they can't help it. How to deal with delusions - common delusions can be persecutary or paranoid - thinking people are talking or laughing about you, that your neighbour is plotting against you, can be delusions of granduer - thinking you are a god or Jesus etc. Another facet of schizophrenia is thought disorder - they may think people can hear their thoughts or that someone is controlling their thoughts or controlling them. They may have difficulty in speaking coherently and fluently. A common feature is auditory hallucinations - they may hear voices that tell them to harm themselves or telling them theyre useless and other negative things and this can be very distressing for them. A schizophrenic may have all or Just one of these symptoms. There are many other symptoms but I've just mentioned some common ones. These patients need medication and properly being looked after.