Rarely. I might plan what I’m going to say in some sort of minor way, but I don’t rehearse an entire conversation. I can’t control what the other person will say. Usually, I’m just thinking about an opening line, if I even bother with that, and then the conversation naturally moves forward.
If I’m going to be interviewed for a job I try to think about how I would answer various typical questions.
If I’m going to be lying, I think about a lot of the possibilities that could come up in the details of the lie. It’s exhausting, and why I hate to lie, and I rarely do it. People who lie often over-talk and answer questions not asked, because they spend so much time ruminating about the details, or because they didn’t plan ahead and stumble through trying to explain. Either way, easy to get caught. All that babble is a liars tell.
I realize the OP isn’t talking about lying, but just mentioning it as something that might be rehearsed.