I've previously talked about voice in ancient mysteries, but I want to add a bit to it, because this subject comes up surprisingly often.
It's a current trend in historical fiction to write in a more colloquial voice than you would have seen in the equivalent fiction of, say, 50 years ago. That's a trend to which I'm contributing.
Interestingly, if you go back in time 100 years or so, the colloquial trend reappears. If you look at Sir Arthur Conan Doyle's excellent historical novel The White Company, which is about mediaeval knights, you'll find the characters speak in perfectly colloquial Victorian English. (Conan Doyle wrote both mysteries and historical novels...he just didn't do them at the same time.)
But in between Conan Doyle and the modern age, virtually every good historical novel speaks like a graduate of Oxford.
That's because in the mid-20th century, virtually every good historical novel was written by a graduate of Oxford.
A classic example (classic in every sense) is Mary Renault, who wrote the best novels of ancient Greece ever. Guess what? She went to Oxford.
I think what happened is, the Oxford voice was so common across quality work that it became the expected standard. Therefore, in the minds of many people, any voice that doesn't sound formal must be in some sense wrong.
Anachronism means putting something into a story that could not possibly be there, because it's a forward reference in time. If Diotima said something clever (as per usual), and Nico replied, "Good thinking, 99." Then you could legitimately reach for a high powered rifle and go in search of the author.
But as long as there are no forward references, then any choice of voice is anachronism-free. Different voices will appeal to different people -- that's a pure matter of taste -- but by no means is colloquial voice anachronistic by definition.
In my case, I'm translating what my characters said in modern, everyday, colloquial Attic Greek, into what my readers understand, that being modern, everyday, colloquial English. I'm afraid Nico didn't go to Oxford. Nor did I, for that matter!
Which raises the other big trend that I want to point out: historical novelists are no longer classics graduates. At least, not necessarily. This is guaranteed to have a big effect. If the author's any good, he or she will be solidly grounded in classics, but not coming from the same homogeneous environment means we'll increasingly read diverse views of the ancient world, that simply could not have happened 50 years ago. The trend to colloquial voice, likewise, will make the ancient world accessible to people who otherwise would never have opened an ancient history book. All in all, it's an opportunity for ancient-period novels to really grow.