I tend to agree with Guilted Lady when it comes to rehearsing conversations in your head. It may all look so perfect when you're mapping out the conversation in your thoughts, then in reality the person in front of you says something you did not anticipate, and boom, the map is gone. Lately, however, I am not as easily shaken by this as when I was younger. I can improvise a lot more easily now.
I don't think this is specifically something that people on the spectrum do, however it's natural that we would worry about such things more, because we are less secure about our interactions with other people.
There's one other thing I'm kind of embarassed to mention here: sometimes I imagine what a conversation would be like with someone I've never met, or in other words someone who doesn't exist at all. Yet, always someone who I could 'potemtially' meet in the future. Like, for instance, a week from now (hypothetically), let's say I'm going on some camp, with a group of people, all interested in nature, but no one knows each other. And I haven't met these people. What I do is, I imagine who the people are I'm going to meet there, and I think about what the conversations are going to be like with them. And I imagine this quite vividly, too. 
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clarity of thought before rashness of action