Schizophrenics do talk to themselves at times, or some do at least. But, they don't talk to themselves all the time, or many don't. The symptoms of those with Schizophrenia vary and different things can trigger different behaviors in those with the condition. What might spur one to talk to himself doesn't mean it will for the next person. And, what makes one talk to themself one time might not the next time. There are no exact rules, in other words.
Schizophrenia is a baffling disease. Some things seem to have no rhyme or reason to them. If a person with Schizophrenia is on their medication and attending some form of counseling, they may not talk to themself at all. It is difficultto say what one will and won't do because after all, that is part of the symptom of the disease itself...the unknown.