In my experience it typically doesn't work, but sometimes it can work and is good to keep your mind busy IF you have some free time. It's just rehearsing, essentially what dreams are, but in daytime and more controlled.
It doesn't work in the sense of predicting the conversation, but do it long enough, or, do it with iterative deepening, and even though you probably won't nail the specifics of how the conversation goes, it WILL frequently prepare you in broad strokes such that the actual conversation's twists and turns won't throw you, even if they do surprise you.
