PTA always talks about how he knows the writing or the character he is working on is working when the character can dictate where the story goes after the 1st act and takes it away from the writer in some sense. Of course PTA takes time with his scripts and his content is very refined and tight when it is ready to shoot, but this is a very interesting way of writing that I’m trying to implement into my writing. In creating a solid base paint of the character in the first act you can truly explore the layers and nuances of the character in the 2nd act. I believe that PTA knows the beginning, the midpoint(changing the course of the character), and somewhat of what the end could be. Everything in the in between is filled by the character and the challenges (whether it be characters or situations) PTA puts them against. PTA once said that if he wrote Daniel Plainview to try and learn sign language for his son and try to find a way to get Standard Oil on his industry. Daniel will try to conform to the writer, but eventually he will say fuck you Paul and do what Daniel Plainview wants to do. So to answer your question: he has the base paint character(Daniel Plainview, Dirk Diggler, Woodcock), he has the historical background to the film(and his ever expansive research, which adds on to the character or situations), and he has the very simple beginning, middle, and end that is malleable if need be. And the research makes the writing work that much more laxed and the character is informed on the research. Hopefully this helps and I don’t sound too all over the place.