One of the things I realized early in planning Best of Three was that, if I wasn't careful, it would seem as though the characters were two talking heads floating in space; all sense of their surroundings would drop away and merge with the aether. And I didn't want this: the surroundings were important to me. So I needed to insert some (semi-random) descriptive sentences, usually no more than a single line, into the flow of the conversation.
These took several forms. Some where Grant repositioning himself in some way. He fidgets if he's uncomfortable, and changes position sometimes as a result in a change in the mood of the conversation. As far as I can tell, no one really noticed this, but that's fine: body language generally communicates on a subliminal level anyway, so that we register "Sue seems a bit irritated" when we see her sitting well back in her chair with her arms folded and her chin sticking out.
Some were background noises in the shop, or events outside the window, and these were themselves of one of three kinds: completely pre-written single-occasion text, meant to be reasonably memorable and thus fixed to happen exactly once in a given playthrough; pre-written but somewhat generic text (such as that about the rain) that could recur; and sentences that were themselves composited from pre-written phrases, so that "You hear" might be interchanged with "In the background you notice" for flavor. The idea was to produce enough variety that the composited items did not become excessively monotonous; there's nothing as dull and, ultimately, irritating as text that repeats the same predictable patterns over and over. Humans thrive on variation.
In doing this, I developed a theory, which is: good simulation of a complex system often requires several layers of prose. The substrate is that which is pieced together from relatively generic bits; then over that there may be a level of additional customization; and, finally, the Unique Items. Enough variation in the way the prose comes about, and the player may be lulled, if not into thinking that you wrote every variation in advance yourself, at least into ignoring the fact that you didn't.