Schizophrenia
The medical condition delirium can cause visual hallucinations, or an unpredictable changing levels of consciousness. Schizophrenia occurs along with obsessive-compulsive disorder (OCD), a disorder in which a person becomes obsessed with certain ideas or actions. However, separating the obsessions of OCD from the delusions of schizophrenia can be difficult. The condition in which a person has more than one personality, meaning they act like a different person at different times, is dissociative identity disorder.  Many people can hear voices like this without being schizophrenic, for instance right before falling asleep. This is called a hypnagogic hallucination. The brain cannot tell them apart from normal sounds that are heard. This is not yet fully understood by science.