One of the more subtle forms of anachronism in historical novels is the intrusion of the author's modern prejudices, both in characterization and narrative.
The end of the 20th century has seen a change in living conditions and social attitudes that threatens to make our ancestors almost incomprehensible. It certainly makes writing a good historical novel more of an accomplishment. Anyone can take their friends and neighbors and mentally redress them in those funny old clothes, then put them through those adventures the old books talk about, but this sort of thin fancy-dress novel does not feel very historical. Often it sounds like a cheap polemic on the evils of the old days, with the protagonists' soliloquies flogging such dead horses as slavery, racial prejudice, sexual disenfranchisement, bad hygiene, religious intolerance, and legally enforced class distinctions. We can all agree, here and now, that these are bad things and none of us would like living in a place where they operated, and assume your readers are equally enlightened.
At the time, though, these practices were the norm, and often considered valuable and virtuous. You take your work far above the mediocre if you arrange your novel to show why these things existed as long as they did, and how they affected even nice people's relations and thoughts, rather than just using them as straw-man villainies.
Take that word, villain. It started as _villein_, and meant a peasant farmer. At what point did it move from simply describing one level of society to meaning someone dishonorable to further meaning someone vile? You will find the latter connotation only once villeins are no longer part of ordinary life, indeed have become so uppity as to be landowners themselves, even rich merchants and bankers who can buy and sell impoverished noblemen. As you might also find, the much earlier tie between dishonor and villainy came from the elite class, who simply did not expect the villeins to think or behave as nobly as they did. No nobleman was surprised when a villein acted like a villein, conniving his way out of work, running from physical danger, taking advantage of those weaker. He might be irritated, even enraged, but not surprised. We, of course, hold everyone up to the same standards of behavior.
When you start looking at the psychology behind words, laws, and practices, you can begin making your Medieval (or other era) hero a man of his times, rather than yourself or your favorite college buddy in velvet and armor. This nobleman has never heard of equality before the law: there are different laws for different classes just as there are different responsibilities. He has never heard of sexual equality or androgyny, but if he runs into a female warrior who can ring his can, he would have to be a stupid bigot even for his time not to acknowledge her ability, and use it if he needs back-up (and most Medieval prejudice against woman warriors came from the Church, not the military class, anyway).
While older books of history may give fascinating details, it is not wise to build your characters on just these views of society. Often, they reflect their authors' Victorian upper-class prejudices, or those of their Victorian sources, more than the actual attitudes of hither ages. Be sure to check your university library for new studies of women's place both in and outside the house in farther ages. The Victorian age may be considered the high-water (we hope) of the dehumanization, limitation, and trivialization of women in our culture, but it leaves its ugly stain still, as it tried to rewrite all other periods in its own image. Women warriors, for example, while rarely common, have such a continuous historical presence that the few to survive the filter of time hint at many more unsung heroines. The number of female merchants, brewers, and other bourgeoisie must also be revised. Before the modern era, it was much less a man's world than later propagandists would like to admit. One charming novel relying on this is *The Maude Reade Tale* by Norah Lofts, about a girl who would rather be a good wool merchant than a fine lady in a castle.
As well, men's attitudes should not be judged solely from those of the often misogynistic monkish chroniclers. Look for diaries and collected letters of the worldly, as well. Their real affection and respect for their wives, and their reliance when away from home on these women to be their trustworthy, competent regents becomes apparent. This includes the merchant travellling all the way across England on business, as well as the prince off on Crusade to Outre-Mer.
Facing the people of your period as they were, not as you wish they were for the convenience of the first easy plot you dummied up out of pieces of other stories, will give you a much richer and more unique finished work. The first plot is really only a seed; the final plot after research may only have a few spots in common with it.
So now, being warned, you will research harder and farther, and not have your ancients disgusted by slavery (how else do you get the house cleaned and water carried from the spring?), your Medievals by serfdom (how else do your fields get tilled, and all those people get fed, clothed, and protected?). You will not automatically force all sympathetic characters into modern political correctness.
You also have to avoid giving your characters the bad parts of our society, in the areas where they were morally superior: those times and places when any stranger must be fed and sheltered, hospitality was a sacred bond passed down through generations, warriors thought killing each other in wars improperly excessive, or any woman could count on the help and protection of almost any man to whom she applied for aid. There is that to learn about different past cultures, too.
Now you have to watch your own self in your descriptions and choice of language.
Take something as simple as housing.
Most authors can work up a head of enthusiasm for the gilding and mirrors of Versailles, the marble perfection of the Parthenon, the cheerful frescoes of Knossos, or the imposing grandeur of Karnak or Stonehenge. Getting down to the everyday, though, most find it difficult not to refer to the average mud-brick house with no running water, no WC, not even a fireplace, in terms of other than pity. As for the Gaulish barbarian, their wood and thatch circular huts are often called hovels.
YOU know how nice a house can be: electric lights, wall-to-wall carpeting, refrigerator, stereo, spa tub. To your Gaulish characters, however, their houses are the best that technology can devise and far superior to a hide tent, a brush lean-to, or sleeping on the open ground, which are their only alternatives. By keeping cattle on one side (these were not little huts), the whole place stays toasty warm in winter. Sure, *you* would feel like you were living in a barn, and one not mucked out often enough, but to your Gaul, the smell of cattle dung is ubiquitous and unremarkable, like the probably unnoticed smell of plastics and formaldehyde resins around your dwelling.
Your Athenian coming home from a festival on the Acropolis will beam with pride at his comfortable, two-story home of plastered mud-brick and wooden pillars, its outlet drains to the alley, its abundance of bronze tripods to hold lamps and braziers that provide both light and heat. He has never imagined a chimney. To him, the smoke curling around the ceiling is as normal as the idea of light bulbs being hot is to you.
So in your descriptions, unless the viewpoint character is a time-traveler, it would be out of place to dwell on the difference in comfort and convenience between there and here. Your characters have no conception of the world in which you and your readers live. They are more likely to notice the best points of a new place or experience, and never mentally comment on the unremarkable in negative terms, unless they are reinforcing their own image of someone who lives differently as a social inferior.
If at the court of Versailles no one takes baths, then everyone is no more offensive to their neighbors and lovers than you are when scrubbed and sprayed to meet the world. It is normal. In your writing, you should not notice it any more than they do. Notice distinctive perfumes, ignore as they did what must have been the underlying ammoniac ripeness of stale sweat. Otherwise, you have to go through silly, distracting gyrations to make your protagonist the only person in the place who bathes daily, besides finding a place for them to take the bath and water to take it in. Running water still consists of servants with buckets or jugs climbing several floors. That, or while you may set a mystery there, you certainly will have a rough time writing a spicy romance that convinces.
Even when class differences make "better living conditions" available, do not let your egalitarianism automatically run amok. Today, not everyone who lives in an apartment over a store downtown wants a house in the suburbs (listen to the Jackson Brown song, "Downtown"). Not everyone wants to party with the rich and glamorous, or spend their time on the social games of that strata. Most people, when they win the sweepstakes, see to a permanent dwelling place, often just paying off the place they have, and set up a financial buffer against the future. They may buy an RTV and go on permanent vacation, but otherwise become known amongst their same old friends for killer Fourth of July steak cookouts and big Christmas parties.
Certainly a wealthy Sybarite viewed a Spartan supper of black broth and barley bread with horror, but black broth is tasty, filling, and high in protein, and you know what you pay for even part barley loaves in the gourmet bakery. Much of that horror was cultural and class prejudice, that wealthy men should eat peasant food: the Spartans aspired to at least the appearance of a classless society, and ate as simply as they dressed. If your viewpoint is Spartan, then your modern interest in traditional peasant foods, so much healthier than haute cuisine, should be used to advantage, to show the reader why the Spartans could enjoy this fare. At the very least, pass over the dinner as one like a thousand others. Your Lakedaemonian suffers no longing for the burgers, berry cobbler, or Caesar salads he has never experienced, and will never invent.
There is no proof that the average serf would want to live up at the castle. Up at the castle, they insist you only relieve yourself in a few limited places, instead of anywhere nearby where you aren't right in someone's face. The toddlers can't squat wherever they like outside (that's why they kept small children in short smocks with bare rumps). You're always under the eyes of Them Folks, and can't scratch and swear as you please. You have to go to Mass every blessed morning (those castle chapels and chaplains were not just decorative). Besides, men at the castle are almost all warriors. You can get killed by that war stuff! Serfs don't have to go to war, can run away and hide if a raiding party strikes, get to pig out at the lord's expense at various holidays and as part of some work details, and can build up a certain cushion of life by working harder in their own gardens or by selling or trading handicrafts.
Unless they have an unusually monstrous seigneur, the life is not unbearable, especially for the illiterate, uneducated masses who whistle while they weed and know they are more likely to reach Heaven than Milord is. If they know they have enough for dinner, they are content. They have no television or glossy magazines telling them their lives lack satisfaction via consumer goods. They do have clerics telling them their way of life is more Christ-like. The most they want is maybe a step or two up the ladder, to be a village headman, but since most material possessions, including the house, are homemade, those who do without are making a choice to not work, to not cut wood to tie into a bedstead but sleep on a pallet on the floor, to not patch the roof or add on a room. They are not being oppressed by lack of money to hire roofers or pay off the furniture store, or held up by building regulations.
Admittedly, in trying to understand how the average person of the past -- always a peasant farmer; the wealthy and rulers are always a minority -- lived and thought, it helps to have an experience of non-American cultures. As has been said, the signs in the local language asking men please not to piss up against the cathedral are not there to keep the American tourists in line. As indoor plumbing and public facilities become more common, this is less common in Europe and the big cities elsewhere, but in much of the world only a battery-powered radio and a kerosene rather than oil lamp marks the peasant house of today from that of one or a dozen centuries ago. The same applies to peasant behavior. They might wear a T-shirt and shorts or jeans, but they are still driving plows behind bullocks, weeding with a sharp stick, eating pretty much the same thing for dinner every night, and stepping behind whatever bush is handy. The jeans and tee are often dirtier and more torn than their old peasant costume would have been because they require cash to replace, but wearing them rather than the old stuff is considered higher status.
So when you decide to make a place unpleasant by your description, make sure it is an artistic decision, and not an unconscious anachronism.
The third form of hindsight anachronism is the unnecessary air of doom or imminent triumph provided by your knowledge of later history. Your characters do not necessarily know that their way of life is going to collapse in their lifetimes. They also do not know that the revolution they are starting may sweep the world: they can only hope so.
If your story illuminates crucial hinges of history, this feeling of foreordination is almost unavoidable: marching to Leuktra or Gaugamela or Waterloo; booking a ticket to leave England on that new liner Titanic; arriving at Versailles in 1793; getting assigned to the 7th Cavalry under Custer; helping this odd Jew named Simon Peter, who says he no longer is a Jew; and so forth.
On the other hand, heading out for Gaugamela, Darius thinks he is a great and unconquerable monarch, and so do his troops, and you cannot let Alexander's victory to come stain their confidence. In 1780, the peasants of France can be blithely ignored if not germane to your courtly tale, your characters living in the dazzling moment without the least idea that many of them will die violently on the scaffold, especially of the youngest you portray. A suffragist of 1885 can feel completely defeated, beating her head against a stone wall, and must carry on merely out of a conviction of righteousness, not because she knows that someday, inevitably, women will win the right to be full-fledged human beings again.
Your reader is here for an interesting story. If your attitude clearly makes triumph inevitable because of history, there is no reason to worry about getting to The End. Keep in mind how many suffragists spent time in jail, were ostracized by their families, divorced by their husbands and never saw their children again, labored and died, with few or no more rights than which they began. Consider the sorrow and fear of individual early Christians, not so much the martyrs as the survivors in hiding, thinking their religion was close to oblivion. Do not forget that despite choosing the victorious side in a war, your characters can still wind up wounded, dead, or maimed, or economically ruined like many patriots of the American Revolution.
Keeping inappropriate hindsight out is often a matter of revision. Ask yourself, "Why is this charcter so gloomy/assured about the future?" If there is no good reason at the time, edit. Characters should never be certain of success beyond their own determination.
Another method, less easily practiced, is to not research beyond your story date. Problem is, if you know history at all well you know all about the major developments already. It is hard to forget the Roman Empire will not last forever; just try to remember how many centuries it still has to go.
Often, if your story shows famous people before they achieve greatness, you must read about their later lives to know their earlier personalities. In many cases, you have to read a decade or so around in the general area before you can settle on a story date. Of course, if you sit down knowing you want to write about a year in particular, you can virtuously close the reference book when an author steps even a month further.
Research and the exertion of a good novelist's skills will eliminate anachronisms.
Let your characters' thoughts be colored by those of the men and women of the period whose words survive. This means you have to read a lot of old stuff in translation, but you will find yourself enjoying it more than the usual historian's report: it is so live and immediate. If not, you are in the wrong place or period.
As an historical novelist, like any novelist, you must be able to think the characters' thoughts, not your own, from inside their personalities and lifestyles. They are just from a very foreign place, removed in time if not space. They are different from us, and that is precisely why they are so fascinating to write about and read about.