One ought to "Know Your Audience", as they say in all general composition classes. But doing this is harder than it sounds. It is a difficulty I've run into in tutoring, TA'ing, attempting to understand popularization with this blog, and just general explanation of science in conversation. I do not think this is as hard with fiction as it is with non-fiction, in particular, with science. There's a certain amount of distinction that comes with scientific understanding, but simultaneously, so long as we're not talking about the popularly held HARD sciences, or a specialization of the harder sciences, I've come across the notion that it ought to be "Common Sense for Common People" -- at least in my attempted explanations of chemistry.
Generally, I figure people know what atoms are, and molecules, and that the entire universe is composed of them. However, beyond that and the existence of the periodic table, I grow uncertain about the layman's knowledge, as anything beyond that was what I learned in my college courses. I recall specifically going into an explanation about water, one time, and thought it necessary to go over the shapes of molecules, but this gave insult to the person I was talking to, who thought I was just trying to show off my knowledge, and also who thought I, being the "Science Major of Awesome", was trying to belittle him, who was a "Liberal Arts" major (Though I really, honestly wasn't. I value knowledge as a whole, and find competitive distinction between the two classes of knowledge as trivial and silly)
So, through this experience I realized that one has to assume some knowledge, otherwise people take insult, and then they'll shut you out. But on the other side of what I perceive to be a thin line is assuming too much knowledge. You get blank looks, but no one wants to admit that they're ignorant, which I think is especially reinforced by this notion of "Common Sense"-ness that comes with the basic physical sciences (which I would define as anything not commonly associated with hardness, ie, not quantum mechanics, string theory, space related, or drug related). Everything else, from my anecdotal and probably somewhat off perspective, and I am mostly referring to information I learned in my general chemistry and my Physics I class, is treated as if one should just know this common sense stuff, when in fact there's no way one could know all of this "Common Sense" stuff without taking the time to learn it.
So, where do you err? It would seem talking "above" people would be better, because then at least, if they are so inclined, can look things up they don't understand. This under the assumption that the alternative shuts everyone off to learning, which isn't the best of assumptions -- I'm sure some people are patient with reviewing things they already know. It just seems a difficult problem to surmount in determining which path to lean towards (as, ideally, you'll just find that happy medium on the thin line) when you "Know Your Audience", especially when your audience can have varying amounts of technical knowledge.
Because I try to avoid the "Ivory Tower" feel of science, and encourage people to keep up with scientific progress, I think leaning towards the "Insult your Audience" side is better. However, as I am also often still learning things myself, and an explaining them in order to better understand them, I think I unintentionally lean towards the "Mystify your Audience" approach.