The characters should be studied individually, in order to find out why the author brought them on the scene; what position each occupies in relation to the whole; who are the most important folk, and who are brought in merely to render some useful but unimportant service to the story.
Then note how the author keeps the circumstances that surround each character directly proportionate to his or her place in the story. The great deeds are invariably performed by the hero—not by some odd man who appears only in one chapter and is never heard of again. The most striking personality is never assigned to some woman who only has a minor part given her, and who vanishes in the course of a dozen pages, with no further explanation.
In this way assess the value of each character to the story as a whole.
Next study the matter that seems non-essential to you, and decide, if you can, why each episode was introduced.
The Use of Secondary Matter
At first glance you may think that much of it could be done without, and would make no difference whatever to the story, beyond shortening it, if it were omitted altogether.
This is perfectly true of poor work. The unskilled writer will pad out a MS. with all manner of stuff that has no direct bearing on the plot. There will be conversations that reveal nothing, that throw no lights on the characteristics or the motives of anybody, and are obviously introduced merely to fill up a few pages. There will be incidents that in no way affect the movement of the story, that add no particular excitement or interest, and carry you no nearer to the climax than you were in the previous chapter.
But the good craftsman wastes no space on unnecessary talk, even though certain scenes and episodes may be of less importance than others. He knows that secondary matter, such as descriptive passages, dialogues, interludes and digressions are necessary in order to "dress" the framework and give it something more than bare bones; they are also needed to give variety and balance to a book. Some incidents that may not appear to be vital to the story, are introduced to break what would otherwise have been a monotonous series of events; or they are put in for the purpose of giving brightness and a picturesque element as a contrast to some sorrowful or gloomy occurrence.
Minor Details can be made to serve Two Purposes
If the book be written by a master, each character, each conversation, each incident, each descriptive passage, each soliloquy is introduced for a specific purpose; nothing is haphazard, nothing is merely a fill-up.