The Bicameral Mind really is a fascinating theory. I wish there was more evidence for or against it either way. The absence of text on free will does not really convince me, because there might be a number of reasons for that and there is not much text anyway for the bronze age and earlier. But it sure would explain all kinds of religious experiences and texts, so who knows.
That when they did act, it was a big booming voice inside their head that they took to be the voices of various gods and deities, guiding them through life via micromanagement.
The wikipedia article does not really do it justice.
I wish I could dismiss it as woowoo nonsense. But far more recently, there are credible reports of circumstances where because a bunch of people were dancing, those who came near them and noticed started dancing uncontrollably themselves, for days on end. Some even dying of exhaustion.
The software that is the human mind is buggy as shit, easily put into bad states, and networked enough that glitches are apparently contagious.