@Shevek, this idea of "over-preparing" for conversations, in my experience, depends upon the conversation you are preparing for. For example, I know that once an important one-on-one conversation (a meeting with your employer, your therapist, your doctor, etc), or even if I am preparing for a presentation or lecture, I need to anticipate that someone is going to ask some clarifying questions. The last situation you want to put yourself in is the
"I don't know, but I will get back with you." scenario, because it diminishes your credibility. I like to delve into some background research and know the topic well enough to have the ability to dig deeper into the topic, if needed. Some people just want the basic facts and get right to the point in as few words as possible. Some people are going to challenge you and you had better know your stuff. Another example is my advice for people who have difficulties articulating their thoughts (like many autistics, myself included), so preparing something written ahead of time, some notes, key points, (for you and the person you are speaking with) knowing that, in the moment, the other person could go off on some sort of tangent, or constantly interrupt your train of thought. I can use that tool to redirect the conversation. I can run a conversation or lecture in my head, repeatedly, for days, and then totally mess it up when it comes down to actually presenting my thoughts verbally.
"Arrrggghhh!!!!" Horribly frustrating. Notes are a handy tool to fall back on.
So, it's not as if you are "in control" of the conversation at all times and can, though mental telepathy, anticipate or direct anyone's thoughts, perspectives, and words at any given time. It's nice if you can keep that control, but that's not reality in most cases. Someone is going to "throw you a curve ball" and you just have to pivot for a moment, then try to redirect the conversation back to your original point.
Now, yes, there are then the vast majority of every day conversations that we get ourselves into that are random and can go anywhere. In these situations, the thoughts sort of have to be spontaneous and are dependent upon your "wit" and ability to quickly recall information.