Writing period is a hard needle to thread. There is a misconception even among some professionals that if we're in the past, everyone speaks with great formality. Some of the early adaptations of the Brother Cadfael novels are downright painful that way:
Monk: I am Brother Petronis, cook to this house. It is from me you will come each day to collect your master's daily fare.
Servant: I know my duties. Today my mistress has a dinner prepared. She begs only a little sage and basil to season her dish.
The same mistake was made for a different reason in HBO's John Adams series. Pretty much all of John and Abigail's dialogue is taken from their letters. The letters are a very famous and illuminating look at an extraordinary couple, and our understanding of these two people and their marriage is the better for their having survived. But they are letters! Nobody talks like that when the person is standing there next to them. Nobody talks to their spouse like that in bed. They obviously meant to do a good thing in using the letters for the dialogue, but instead it reduced this amazing vibrant relationship to an artificial dead thing.
The best way to illustrate what goes wrong is to turn to badly conceived fan fiction. We all know the dynamic and it's been discussed exhaustively elsewhere: Ditzypoof has a story so monstrously wrong, say Batman/Ra's al Ghul, a love story. This pairing violates character so completely that no one with 10 firing synapses can possible look at it and see that character as "Bruce." It's something the writer is calling Bruce and treating as if he's the guy whose parents were killed in Crime Alley and honed himself to become Batman, but he's saying and doing things that violates his core programming, which is not possible. When you eliminate the impossible, whatever remains…Q.E.D. that's not Bruce. This is where Ditzypoof inserts what she thinks justifies her pairing: Bruce says "My City", Ra's calls him "Detective", so see, it's them. In her delusional psychosis, she thinks the catchphrases make it so.
THAT'S the problem with egregiously formal dialogue. It's having them use archaic construction and vocabulary to establish "we're in the past" the way "my city" is supposed to establish that Bruce-named-thing in the badfic is Bruce Wayne. At the same time, the way the very behavior and the ideas expressed prevent him being Bruce Wayne, the archaic dialogue prevents the period characters from being HUMAN. We live and die with Frodo, Aragorn and Galdalf because they are human in the "like us" sense. We love these characters and care about what happens to them because of that humanity. If they become cardboard cutouts who just move the costumes around under the lights, who gives a damn?
As with so much, it's the hard part that makes the difference between the good stuff and the bad. You've got to understand the IDEAS behind the words to know what to do and use when. It's not using the words of 1183 that makes a character viable as a credible being from the middle ages, it's the thoughts and attitudes those words express. An introduction is a big thing. That will be a very formal exercise. Being invited to someone's home is a big thing and that will use formal construction. But whispering to your friend that you think his son is diddling the maid, or the blowhard you both hate is stealing from the treasury, that's going to be expressed in pretty much the same way we do today.
The reverse is also true. It's painful to see ludicrously modern ideas in 16th century heads that the author thinks they can justify by putting it in old world terms. But that's an issue for another day.






