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.