I wouldn't say I'm shy so much, but I'm very reserved during the first time I meet people. I prefer to listen and see what people are about before opening up to them myself. I feel I've always been a good listener and I can usually read into what people say quite well. If I'm unsure how to judge something I'll usually try to say something humorously ambiguous because I'll either inadvertently 'hit the nail on the head' or make them laugh, giving me information about them either way. I'm reasonably introverted and often spend a significant amount of time on my own. I often imagine conversations I'm going to or might have almost obsessively. Trying out phrases with myself and gauging reactions or rehearsing responses to certain expected and unexpected questions. I'll also think of ways I can subtly steer a conversation towards something I want or have to say. I tend to predict correctly so it makes conversations quite easy to have but if it goes off to a completely unexpected route it can sometimes take a hit. Also, if someone just tries small talk with me I will shut off almost immediately. I absolutely deplore it. However, when it comes to being with friends and family I often think of "You can tell you're close with someone when you can enjoy their company in silence".
It sounds like you're confident in your interactions. While being present, writers also tend to see "the story" in a situation and, as you say, rehearse responses. I'm wondering if your statement that you "imagine conversations" feeds your writing, both analytical and creative. I don't imagine or rehearse conversations I might have, but if someone is silent (spousal unit perhaps), I might supply both sides of the conversation. (I think I'm kidding, but I might not be.)
Writing dialogue is difficult because you have to be really concise and on point compared to the often flabby nature of real human interaction. It's something I'm practicing though, along with my overuse of adjectives. I know what you mean about supplying both sides of the conversations. When you say something that you know they should have a response to yet they're silent, you often end up suggesting the answers you expect of them in order to pry them open. It doesn't often work though, haha.