Since my mysteries take place at different time periods in the past, one of my personal “research” assignments is to study the language of those times. The style of language is important, certainly, in the narrative, but, absolutely, in the dialogue.
The flow and rhythm of the narrative helps set the tone for the story in the past. The dialogue should be close to language at the time, although revised enough so the modern reader can understand it. Here’s a combination of narrative and dialogue from Pure Lies, about the Salem witch trials of 1692:
Sixteen-year-old Felicity thinks: “Was all this a grand deception? A vile and sinful imposture? Could her own friends fabricate such a cruel and terrible scheme? Procter’s words came back to her and filled her with a morbid sense of dread. ‘They have concocted the devil out of the stuff of nightmares and, more, out of taedium vitae.’”
When it is useful to the story, I use the actual language written at the time. For example, here are some words from an arrest warrant for Susannah Martin:
“You are in their Majests names hereby required forthwith or as soon as may be to apprehend and bring (before us) Susannah Martin of Amesbury in the County of Essex Widdow at the house of Lt. Nationiell Ingersalls in Salem Village, in order to her Examination Relateing to high Suspition of Sundry acts of Witchcraft donne or Committed by her upon the Bodys of Mary Walcot Abigail Williams Ann Putnam and Mercy Lewis of Salem Village of farmes.”
Believe it or not, many citizens of Salem were literate at that time, simply because they were required to learn the Bible.
In my research, I read as many books of the time and about the time as I could to get a sense of the proper language but I often had to look up the date which many words or phrases came into use. For instance, I wanted to suggest that the “afflicted” girls were bored and cried out against their neighbors for sport. However, the word boredom didn’t exist at that time. Interesting, eh? It actually came into use around 1852. The word sport, however, dates back to 1582.
The modern story in Pure Lies takes place in 2006 and, for the most part, didn’t present language problems. Although with the constantly changing technology, I had to keep an eye on that as well.
Critique groups and a good editor can be very helpful in pointing out flaws of language in both historical . . . and modern pieces.