Writing historical fiction

Claudius the God, Robert Graves
From the author's note prefacing the book:

Some reviewers of I, Claudius, the prefatory volume to Claudius the God, suggested that in writing it I had merely consulted Tacitus's Annals and Suetonius's Twelve Caesars, run them together, and expanded the result with my own "vigorous fancy." This was not so; nor is it the case here. Among the Classical writers who have been borrowed from in the composition of Claudius the God are Tacitus, Dio Cassius, Suetonius, Pliny, Varro, Valerius Maximus, Orosius, Frontinus, Strabo, Caesar, Columella, Plutarch, Josephus, Diodorus Siculus, Photius, Xiphilinus, Zonaras, Seneca, Petronius, Juvenal, Philo, Celsus, the authors of the Acts of the Apostles and of the pseudo-gospels of Nicodemus and St. James, and Claudius himself in his surviving letters and speeches. Few incidents here given are wholly unsupported by historical authority of some sort or other and I hope none are historically incredible. No character is invented. The most difficult part to write, because of the meagreness of contemporary references to it, has been Claudius's defeat of Caractacus. For a plausible view of British Druidism, too, I have had to help out the few Classical notices of it with borrowings from archaeological works, from ancient Celtic literature and from accounts of modern megalithic culture in the New Hebrides, where the dolmen and menhir are still ceremonially used. I have been particularly careful in my account of early Christianity to invent no new libels; but some old ones are quoted, for Claudius himself was not well-disposed to the Church and derived most of his information about near-Eastern religious matters from his old shool-friend Herod Agrippa, the Jewish king who executed St. James and imprisoned St. Peter.