What? It's never occurred to me that this isn't entirely normal, I've done it all my life. I thought people without an inner monologue were the unusual ones.
I think there is a difference between having an inner monologue, and imagining a conversation with another person in your head.
I'm not wading into the schizophrenia part, but inner monologue doesn't necessarily imply constructing a fully formed sentence you then repeat to yourself.