Then, insofar as fiction instructs as well as delights (the two functions any medieval writer attributed to stories), it does not so much instruct the intellect (though it can) as the sentiments. To prevent C. S. Lewis's Men Without Chests -- not to argue that courage and loyalty are good as to show them in their loveliness. Even if you want to demonstrate an abstract idea, in economics, you have to bring it down to a blood and guts level. Showing, for instance, the deleterious, soul-sapping effects of a socialist economy in living if dreary color. Or, for that matter, showing all the happy workers of the communal farm, who do not suffer the collective action problem. (No rule, alas, that what you are showing must be true. Though falsity has its problems in realism. Hmm. Actually, come to think of it, socialist realism had more problems than the falsity of its teachings, in that it allowed no conflict. One writer was actually asked why his horse wearily plowed the field; why was it not cheerfully doing so, in a lively manner? Critical realism, where they brought to bear fiction on the horrible capitalist world, often turned out better.)
Of course, once you've brought it down to that level, you can then often bring in the intellect as the characters will often care about the governing principles, and they relate to the stories conflict. And you can bring it in exactly and precisely to that extent. A story exemplifying a dreary, wasted, stultified life, in which a character is chewed up and spit out by a welfare state without ever fathoming what has happened to him does not allow him to talk about his life clearly, because half the point is that he is prevented from realizing how his life has been confined.
Bringing in other characters to talk about it is dangerous. Even if they care. The dreaded info-dumping lecture is not relieved by its being delivered even by a monomaniac on the subject; though sometimes you can slip bits in while your POV character is dreadfully bored and conspiring to escape, it's not much. A larger cast can sometimes allow it, and sometimes foul up the main story in the process.
Hmm. To borrow another old trope, ancient rhetoricians distinguished between the high mode, which persuaded; the middle mode, which amused; and the low mode, which instructed. (Some of them claimed there were appropriate to high, middle, and lowly objects respectively, but others pointed out that was silly. If you wanted to move people about lowly matters, you used the high mode, and if you wanted to instruct them about even the highest matter, you used the low mode.)
Fiction is not allowed to use the low mode; only non-fiction. Which is why complaints about info-dumping often compare it to non-fiction. That which instructs in fiction must also move or amuse.