The same under the skin?

When I performed in Merry Wives of Windsor a few years ago, one of my co-actors was completely stunned by how much the subplot — Anne Page’s desire to marry a man her parents didn’t approve of — seemed like something out of a modern play.
As I’ve mentioned before, it’s difficult to get into the ways people of the past are so different from us: Their fears, their faith, their bigotries and hates. At the same time, they also have a lot in common: The experiences of men in battle have been quite consistent throughout history, as detailed in Richard Holmes’ Act of War.
Trying to find the balance between the two can be difficult, even if you’re not striving for deep historical realism. In the first place, there’s the image you want to create: People in the past slouched, relaxed and used the slang of the day, but many films have portrayed ancient Romans/Greeks/Babylonians standing stiffly and declaiming rather than talking.
People in the past also have nasty attitudes: If you’re going to do a reasonably accurate job, they’re not going to share current perceptions. Hugh, the protagonist of I Think, Therefore I Die, which I’ll finish next month, loathes Catholics as oppressive agents (or dupes) of the tyrannical theocrat in the Vatican; that’s certainly a common English perception for the 17th century, but I felt quite uncomfortable with his vehemence at times. Even if you go back forty years, you can find decent, respectable good-guy types who think the color line should never be crossed or that women should totally stay in their home. The Aztec heroes of de Borchard’s Servant of the Underworld don’t bat an eye at human sacrifice.
I’m inclined to agree that in any time, human nature remains constant (and yes that is a very general observation): We want sex, we scrabble for status, we worry about feeding our children or hiding our secrets. The way it’s expressed, though, may be very different. The modern world may have many people as devout as the Christian stylites who spent months sitting on pillars to prove their faith, but devotion to God just doesn’t manifest that way any more. In the 19th century, an adult courting a 12 year old girl with her parents’ consent was respectable (you could get married at 14 in Victorian England); today it’s a symbol of either neurosis or pedophilia.
I’d like to end this with some kind of useful rule for figuring out How To Do It; as I write so many stories set in the past, I could use one. But I don’t have one, so I’ll just have to work it out as I go along.