Since I, apparently, wasn't going to get any help writing this novel, I decided to take the plunge and try to do it myself. I still had major confidence problems about my ability to write fiction, but I ignored those (as best I could).
I started reading books on writing fiction. They were pretty consistent in how they said to just get started; that most [hopeful] writers fail because they try too hard to figure it all out up-front and they never really get to the writing. They also said, once you really get a story going, that the characters would write the story. WTF! How could the characters write the story? The author has to write it.
I also spent many months reading the scholarly non-fiction works on the topic trying to figure out a sequence of events that made sense for the time frame. And then, finally, I began writing. The first few chapters were, shall I say, stilted, but I was getting through them and I could see, even during those first 60 pages, my writing improving.
Then I was at a point where my protagonist, an ex-Senator of Rome who knew a great deal about the Jewish people, was arriving in Rome for an extended stay. I wondered what should happen at this point and it was obvious that the Senate would invite him to come give an open speech to the senate about the situation in Judea. So, I had to write an entire scene that I wasn't expecting to write and had to formulate a speech to the Roman senate. That scene wasn't in my plans or outline anywhere.
After I finished the scene, I remember thinking this is what they meant by the characters writing the story - the fictional situation in the book presented, to me, what needed to happen. On the tails of that thought was the thought that I was becoming a "real" writer.
From that point forward, I let the details of the story develop more on their own and trusted the ideas I had for what needed to happen. Of course that was all while trying keep with the history of the times. It was also within the framework and plot that I had already established and documented. It helped a great deal to be intimate with the story before writing it. It also helped that I had done profiles and a history for the main characters. Moreover, that I had become so familiar with them that they felt like friends. Once all that was in place, letting the situation and the characters (past and personality) drive the story felt natural.