That said, I also have some new writing theory to journal about for the first time in months. This is related to the Dramatica theory of writing, as well as the cult that has grown up around Joseph Campbell and _The Writer's Journey_, a formula for writing mythic fiction which is very rigid, but effective if you want to write that kind of thing. I, however, don't.
So, here's Teh Sunandshadow's mutant version of the theory of the interface between characters and plot:
The terms protagonist and antagonist are useless. Every primary character is capable of acting in both protagonistic and antagonistic ways, especially when that character has strong internal conflict, filling both roles at the same time. (A vivid example of this is the character Raskolnikov in Dostoyevsky's _Crime and Punishment_. (Note: a character may actually be half of a person (Dr. Jekyll and Mr. Hyde are 2 characters), more than one person (twins, a mob), or a personified inanimate object such as nature.)
It IS useful to distinguish between primary characters and secondary character, since they are developed and used in fundamentally different ways. The essential difference between them is dynamism. Secondary characters do not fundamentally change during the course of a novel. Primary characters, while they may in the end decide not to change, struggle throughout the novel with forces pressuring them to change and other forces holding them back from changing. Thus, every primary character (assuming that character is not purely a hero or a villain) has an internal story as well as participating in the external story, while secondary characters participate only in the external story. (The Dramatica theory refers to these storylines as throughlines, although it maintains that each story has exactly 4 of them.)
A novel is composed of an external storyline which contains all the characters, plus an internal storyline for each primary character, plus storylines between each pair of primary characters - so if you know how many primary characters you have, you know how many storylines you will need to weave together to make your novel (although authors may choose to leave out some that seem redundant or unimportant).
So, # of plot strands = 1 + P + (P-1)factorial
Since every story must have at least one primary character, the minimum number of plot strands is 2. An example of a novel of this type is William Golding's _Pincher Martin_.
My own novel has 4 main characters (M, A, L, and R). So it has 1 + 4 + (4-1) + (4-2) + (4-3) = 11 plot strands. These plot strands are as follows:
External = Family vs. Society
Internal = M vs. self, A vs. self, L vs. self, and R vs. self
Interstitial = M vs. A, M vs. L, M vs. R, A vs. L, A vs. R, and L vs. R
Of these, M vs. R and A vs. L are relatively unimportant and could optionally be left out.
Now, these plot strands can't all happen at the same time. In point of fact, a scene may have only one plot strand, and one would be hard pressed to include more than three, one of each type (external, internal, interstitial), in a scene. So, The weaving of a novel is a process of choosing which plot strand(s) to explore in any give scene, while making sure that all the plot strands get explored, hopefully with some kind of rhythm resulting in a climactic bang at the end, throughout the course of the novel.