This gets into a lot of the confusion about what schizophrenia actually is -- a lot of old movies portrayed it as multiple personality disorder (or the clinical term, "disassociate identity disorder"). It's true that both often involve hallucinations and voices in your head, but Hollywood always writes these illnesses purely according to whatever will make the freakiest plot twist (sadly, I do not go to sleep and wake up as Brad Pitt, or even Edward Norton).
20th Century Fox
"I am Jack's inaccurate plot twist."
The key difference is that for me, these voices never "take over" -- I'm always conscious and in control of myself. They can make it hard to focus when real people are trying to talk to me, but otherwise, I'm more aware of my surroundings than what you'd expect based on what you've seen in movies. For example, there's a scene in Girl, Interrupted in which a schizophrenic character who'd been badly burned in the past suddenly realizes the extent of her disfigurement. That movie is based on a book based on a true story, so I don't know if that really happened, but I assure you, I know what I look like (fabulous, in case you were wondering).
There are levels of severity, of course, just like with any illness -- some sufferers are incapable of even basic communication, so it's literally impossible to know what they're experiencing. But neither version tends to show up in movies -- this kind of mental illness is usually an excuse to have the main character suddenly realize they were the killer all along, or to create an ambiguous Black Swan or American Psycho situation. But that brings me to a crucial point ...