Capitalization, Grammatical Fine Points, and Trusting Readers
In Harald, for example, "commander" was the title used in the Imperial army for an officer commanding a legion, an army, or a fortification; like "captain" or "commodore" in the British Navy two hundred years ago, it was something between a rank and a job description. But "the Commander" was also how one particular officer, widely regarded as the best, was routinely referred to, whether or not the speaker happened to be in an army that that officer was currently commanding. A modern equivalent would be "general" used as a rank, and "the General" used to refer to one particularly prominent general.
Strictly speaking, as I understand the conventional rules of capitalization, "commander" should be capitalized when it is the nickname of a particular officer but not in the more general context. Both I and my editor were concerned that readers who did not pick up on that distinction would assume we had simply been careless and inconsistent about when to capitalize. We ended up going with "commander," uncapitalized, in both cases.
I have encountered the same problem in Salamander, the (very different) fantasy novel I am currently working on. A central plot element is a sort of magical chain reaction that lets one mage get control over the magical power of a large number of others; I (and the mage who invented it) refer to it as "the Cascade." Since it is a proper name it ought to be capitalized. But the same word appears repeatedly in closely related contexts, describing the effect that the spell produces: "As the cascade started" would mean, not "as the spell was cast," but "as the chain reaction began to happen," and one can imagine a number of other such uses. I am again in two minds as to whether to capitalize selectively, which I think is grammatically correct, or uniformly, one way or the other.
In part this is a question of how much I trust my readers to understand what I am doing. But it also involves a different question: How much am I responsible for nudging my readers towards, rather than away from, correct grammatical usage.