Dialogue and Historicals

Depends on the period. When I set something in the 19th century, I try to use some period slang and avoid anachronisms. But I don't try to capture the exact speech of the day, because it would sound stilted to us.

I have a couple of Medievals, though, and in the 14th century English was essentially a different language. I assume that what I'm writing is a modern translation of what my characters were saying. At most, I might look to Chaucer for an occasional oath to add color, and try to avoid obvious modern slang. But I don't bother to look up individual words and see if they were in use at that time as I do with my Regencies/Victorians, because I work from the premise that most of the vocabulary was different.

Interesting.

This is another topic where writers fall into different camps. Obviously for prehistory periods you can't look up the period writing! Plain English holds for them and referring all your written images to the natural world helps too.

For my 17thC writing I make sure I read all the period diaries and letters once a week as I write. But I do a quick check up on words (I use the online etymological dictionary) as I write. It helps keeps the right feeling to the words my characters speak.
There are expressions and idioms in the diaries and letters which can go direct to a modern reader. Some of it can't and needs a filter into plain English.
Amazingly most of our plain English words, especially the Latinate words, have been around since the Romans, so to speak, and can be safely used.

The biggest problem is not so much the words to use in dialogue as how to convey the attitudes. When Death sat on your shoulder daily, when God, via your church, controlled your life and what you may and may not do and say, when the weather controlled your food and activities, when the day's light and the moon's light were vital to the structure of your life, you are not someone a modern reader can easily communicate with.

As far as editor and publishers go these days it's plain English they prefer. Plain English is accessible, readable and yet if used with a FEW period expressions and period inversions it sounds right!