Paddy Chayefsky:

It is a common illusion that dramatists sit down and preconceive a detailed biography and character study of each character in the script. To a professional writer, this would be a palpable waste of time. A writer usually starts off his thinking with a rough feel of the character absorbed from some experience in his own life. It is inevitable that the pre-conception of the character will change 1000 times during the course of construction in order to satisfy the demands of the story line. Drama unfortunately will not allow for the complex, contradictory impulses that constitute real-life people. The best you can hope to achieve in dramatic characterizations is an essence or basic truth of the character you had in mind when you started. Holding onto a pre-conception of a character is one of the worst stumbling blocks in the construction of the story. It keeps you from openly accepting or even looking for the correct incidents to tell your story because those incidents are not suited to the preconception of the character. In the end, you will have to change your preconception, but only after you’ve gone through hours of despair and excessive smoking. Generally the characterizations are devolved from the incidents of the main story and not preconceived.

It is, of course, not as simple as all that. That is the difficulty with theorizing about such things as writing. Obviously the characters are not deduced merely from the incidents – because the incidents are derived from the characters. Writing is such a confused business of backing and filling, of suddenly plunging into the third act while you are still pondering the first act. Writing is unfortunately an emotional as well as a mental trade, and the simplest steps in logic are obscured by the writer’s own fears and anxieties, most of which he is unaware of. The writer’s mind may flood with images or run raspingly dry so that he cannot pull himself out of a fruitless line of thought for hours, even days, sometimes never, and the script has to be abandoned in the middle. Nevertheless the overall logic of characterizing the people in the drama is one of devolution rather than preconception.

— Television Plays, by Paddy Chayefsky, 1955, pp86-87 of “Printer’s Measure: A Construction.”