The rough draft is put down and the idea that was kicking around in my head is fixed in place.
It isn't just a case of writing a story, however. The antagonist and protagonist need a reason to be at each other's throats.
What is the motivation? When writing historical fiction, it isn't always a case of looking up the past. Most unpleasant interactions wouldn't appear in a history book.
Reading the history is one thing. It's up to the author to read between the lies, to envision two people squaring off. Human nature doesn't seem to change from year to year. People will forever be jealous of a rival's success. They'll begrudge another's accomplishments, and sometimes they'll do what they can to pull the adored one from the pedestal.
There was the motivation, right in front of me. The antagonist in real life was an ambitious man, stepping on the backs of others to reach the top. Interaction with the protagonist took place early on, before the opening of my story, but the tension was set and bringing it out was as simple as inserting a few lines of back story.
No need to go into a big long exposition about how their enmity began. A short and sweet reference to an act in the past will be enough to set the stage and put the conflict into context. Of course they hate each other, the reader will say. It will make sense.
The logic comes from historical fact, coupled with a little imagination. As an author, you have to observe human behavior, and sooner or later you'll create a character who fits someone you've seen or heard before. Stealing from experience, you'll create a realistic, believable person.
You might think you're veering off on a tangent, wasting valuable time. Like a movie scene of five minutes that takes a week to film, you spend a lot of time reading and gain little more than a sentence or two, but that short piece of back story could make the difference between a cardboard character and a genuine, flesh and blood individual the reader can understand.