Why can’t the Main and Influence Characters both grow and change?
I don't understand why the Main Character and the Influence Character can't *both* grow and change in a story, for instance in a story of a marriage. In order for the marriage to be successful, chances are that both characters need to change.
Both the Main Character and the Influence Character do grow over the course of the story. However, character growth is different than fundamentally changing your outlook on an issue. The change/steadfast issue concerns the characters' resolve. The growth issue concerns the direction of the growth: out of something or away from something (stop), or into something or toward something (start). Besides, a "marriage" can have, figuratively speaking, a life of its own complete with its own central issues -- issues that are related to but different from those of the Main Character and Influence Character. In Dramatica, we call this relationship between the MC and the IC the Relationship Story Throughline.