Adam Roberts (yes, again):

My problem is not that [Miles Cameron’s Against All Gods] gets this or that specific historical detail or mood wrong; it’s that it doesn’t really engage with ‘history’ at all, despite pretending to do so. Its characters’ sensibilities are modern, its gods agents in the imagined world much as its mortals are — the gods are more powerful, though ‘power’ is rendered here only in terms of the ability to overbear, with violence or words — all potestas, nothing of auctoritas. There’s nothing in these gods of the numinous, the transcendent, nothing of the strange, the awe-inspiring, tarrying or resplendent. This is not a dimension that Cameron reproduces in his imagined Bronze Age; although for ‘actual’ Bronze Age human beings, in their porosity of subjectivity, it was a crucial and wondrous and terrifying aspect of existence. The characters go about their various plot-driven actions, and the storylines are punctuated by interludes of purely somatic intensity (the violence, the fighting, the sex) that do nothing to estrange, to capture or embody the wonder and strangeness of the past as such. 

Great post by Adam. I would just add that precisely the same problem afflicts most SF, which cosplays an imagined future as fantasy cosplays an imagined past (or past-like secondary world). As someone who has toyed with the idea of writing both fantasy and SF, I have always believed that this is the greatest challenge: How to avoid writing characters who are people exactly like me, only placed in a different natural, cultural, and technological environment? But people who are situated in radically different environments develop in wholly different ways: each Lebenswelt generates its own distinctive range of cultural and personal possibilities. Trying to imagine my way from (a) the possibilities, the options of mind and action, available to me in my Lebenswelt to (b) what someone formed in a radically different environment might experience … well, that’s astonishingly difficult. (Indeed, I have felt this challenge so strongly that I’ve never completed anything in either genre. The problem defeats me.) 

One writer who has attempted to think through these problems, though primarily in one novel and with one character, is C. S. Lewis. (He attempts a similar act of historical imagination in Till We Have Faces but without the explicit contrast to our own world.) The novel is That Hideous Strength and the character is Merlinus Ambrosius, who is awakened from 1500 years of sleep into mid-twentieth-century England and is puzzled by everything he sees. For instance: 

"Sir," said Merlin in answer to the question which the Director had just asked him. "I give you great thanks. I cannot indeed understand the way you live and your house is strange to me. You give me a bath such as the Emperor himself might envy, but no one attends me to it; a bed softer than sleep itself, but when I rise from it I find I must put on my own clothes with my own hands as if I were a peasant. I lie in a room with windows of pure crystal so that you can see the sky as clearly when they are shut as when they are open, and there is not wind enough within the room to blow out an unguarded taper; but I lie in it alone with no more honor than a prisoner in a dungeon. Your people eat dry and tasteless flesh but it is off plates as smooth as ivory and as round as the sun. In all the house there are warmth and softness and silence that might put a man in mind of paradise terrestrial; but no hangings, no beautified pavements, no musicians, no perfumes, no high seats, not a gleam of gold, not a hawk, not a hound. You seem to me to live neither like a rich man nor a poor one: neither like a lord nor a hermit.”  

The novel I think is flawed, but this is quite brilliant, and I wish the world of fiction had more like it. One reason there isn’t: Merlinus is to his modern interlocutors a thoroughly appalling character, who quite readily suggests that a woman who has not behaved in the way he thinks right should be beheaded, and then is befuddled by the response this opinion receives. (“The Pendragon tells me … that you accuse me for a fierce and cruel man. It is a charge I never heard before. A third part of my substance I gave to widows and poor men. I never sought the death of any but felons and heathen Saxons.”) And to create a character so alien to our readerly sensibilities is a risky thing for a storyteller to do; perhaps Lewis was wise to do this only with a minor character. 

After all — and here the imperatives of historical imagination may run contrary to the imperatives of good storytelling, readers do typically want to … well, we have different words for it: people used to say that they like to identify with characters, but now they’re more likely to say that they find characters relatable (or not). This is an impulse that I don’t wish to discourage: as Edward Mendelson says in his excellent book The Things That Matter, “A reader who identifies with the characters in a novel is not reacting in a naïve way that ought to be outgrown or transcended, but is performing one of the central acts of literary understanding.” But, I fear, the more seriously a writer takes this reaction the more constrained that writer will be in historical imagination.