Lots of people confuse dissociative identity disorder with schizophrenia. They're actually two very different disorders.
Schizophrenics do not have multiple personalities at all. They have, in general, confusion about what is real and what is imagined. In schizophrenia, delusions are common. Delusions are beliefs or thoughts that are not based on anything real or justified. A "paranoid schizophrenic" may suffer from delusions of persecution, for instance. Some sufferers also hallucinate -- they see or hear things that aren't really there.
There are, however, a few common threads between dissociative identity disorder and schizophrenia. Both may hear voices in their heads, and may withdraw from social interaction. Both are often accompanied by other mental illnesses, like depression or addiction. And both disorders carry an increased risk of suicide.
Next, myth 2: "They're faking it."