It is often overlooked now, but one of the many crazy things that George Lucas insisted on in Star Wars was that his sets have a dirty, lived-in feel to them. He didn't think that space would be any more neat than life on this planet was and he thought that imagining it to be so was simply another reason to find his movie difficult to believe in.
Of course other movie-makers got what he was doing and employed the philosophy almost immediately. Grime is a regular element of all science fiction and fantasy work now. It's also present in the other genres as well - dramas, comedies, westerns, etc. It has made films more believable along with several other advances in cinematography.
Historical fiction, however, should have an additional bit of realism in the look. Yes, even the 1980s with their social smoking was much dirtier than today, but the further back in time you go the less sanitary life was and the more diet, diseases, skin irritations, and war injuries effected the look of the individual.
The consumption of water has drastically changed the way our skin ages. Look at pictures from the nineteenth century and compare them to people of similar ages today. They looked old much faster. Our more careful intake of sugars and fatty foods has allowed people to remain more slim than we were decades ago while our more complete diet has allowed the typical human to grow taller over the last few centuries. If Arthur was six feet tall he was a giant in his age.
Lesser strains of smallpox circled the Old World at regular cycles, and these would have regularly left scars on peasants and warriors alike. Several other communicative diseases would have left their marks as well.
Cleanliness was not as well developed as it has become in the last century, so that skin blemishes and skin diseases were probably much more common than they are today. During the early medieval period it was believed that the oils a body produced were holy things not to be washed away very often so that a person generally lived with their sweat for weeks at a time.
War was also different. After the Civil War, it was common to see amputees who had lost their limbs when a cannon ball blew it away or an infection had forced a surgeon to cut off the entire limb. Such may have been common in the ancient world. A person hit with an arrow might have lived with it for the rest of his life. A veteran might have dealt with cumulative concussions (such as the modern football player has). All these elements of the historical world could be added to any performance of the past.
Of course there would be some drawbacks. Colin Farrell's Alexander spent his life drinking and fighting. Legend has it he was never injured in battle until he came to India, but it's unrealistic to believe that he didn't come down with some disease - social or viral - during his travels from Egypt to India. Likely by his early thirties he had scars from one or more of them as well and his skin must have looked dried up from all the dehydration too. He did, after all, walk through a desert at one point.
Alexander lived in a time of relative stability, with the Persian Empire still vital and him able to simply make use of an already existing bureaucracy. King Arthur, recently played by Clive Owen, would have lived in a time nearly without science, without the exchange of ideas and knowledge. He would have been lucky to reach thirty, he would have felt like an old man by then. He probably came down with one or more diseases in his early youth and had the scars of his survival for the rest of his life. As a veteran of battles and raids he was likely scarred all over his body. If he never lost a limb he was lucky but certainly he lost fingers and toes.
Perhaps that's one element of historical fiction that we aren't ready for, after all.
(Source: Xiaverse Wikia)