Dialogue is rather tricky in a historical novel. When I was a child, even good writers like Ford Maddox Ford, when writing historical potboilers under his second pen name, would use a sort of phoney 'Tudor' speech that was only half an inch from a boddice ripper novelette. I suppose this was intended to act on the reader's ear rather as a stage set might on the eye. My own judgement is that since we are trying to convey real people like ourselves, we shouldn't add these false filters, but employ straightforward modern speech. If a scene is set in Roman or medieval times, after all, one is hardly going to put the dialogue in Latin, Anglo-Saxon or Norman French. What I sometimes try to do however, when I'm able, is to suggest rhythms of speech from the period concerned, so long as they don't seem too artificial, using words that are either well-known from the period, or that have continued in use with the same meaning until today. With spelling changes, you can even do this with Chaucer's language, and certainly from Shakespeare's time onwards. Until my generation at least, the seventeenth century language and sonorities of the Authorised Version of the Bible were also familiar to most people. Above all, one tries to avoid anacronisms - though I'm sometimes guilty. I once had a character in seventeenth century Europe say that he'd been 'sold down the river' - an obvious reference to American slavery from a later period. Fortunately, it was expunged in the editing.