No responses huh? Guess it's just you then.
I've discussed or tried to discuss this on numerous forums and at in-person workshops. At first many are, "yes my characters do and say things that I would never imagine and I just write it down."
And then when I bring the Hearing Voices mental illness everyone goes silent.
In pursuing this discussion in a few places I've gotten further clarification -- well it's not really like they speak and I hear them, it's more like I imagine what they would say or alternatives and then write it down.
I think perhaps those that claim to actually have characters act independently and autonomously in their minds are actually doing a lot of unconscious processing that they are unaware of .... and/or perhaps they are a bit mentally ill.