Have no idea if it an aspie thing or not, but lol I do this all the time, but trying to stop myself because it is causing me more trouble than it is helping. My husband was the one who got me to realise what I was/am doing. I feel angry when in life it does not turn out how I had invisaged it, which leaves me disappointed. My husband rightly said that I have no right to think for another. I had not realised I was, but I guess he is right and so, when I can feel myself going into the realm of having a conversation with the other person in my head, I try to stop, but it is sooo HARD to achieve and I soon find myself going there again and have to speak out loud and say, stop it, change your train of thought. What is the snag is that it is fun.
As I see it, there is nothing wrong with you having these conversations in your head. You (I) do it for a reason. We are simply mentally walking around the subject, and getting a feel for it's various possible outcomes. The problems come when we overlook this point. We do it so that we can have a better understanding of the responses that we get from others, and so that we can possibly anticipate what might be required from us when we have the "real" conversation.