I’ve always felt that characters are more important than the plot, or the situation you place them in. But I certainly don’t mean to infer that plot is unimportant.
Plot is cause and effect, or the border of actuality. If someone makes your protagonist angry and he considers throwing a rock through the antagonist’s window, it’s not a plot unless he actually does the deed. His actions tell a great deal about his own character and determine whether the readers are going to care about him. The effect of his actions, or the reaction to the broken window, is what actually creates the plot.
A character’s thoughts and emotions cross over the line into action and reaction. But no action in itself can be considered plot if it doesn’t affect the outcome of the story. Plotting decides what’s important to your storyline. If the action doesn’t apply directly to the resolution, leave it out. Going off on tangents only confuses the reader and makes her impatient to get on with the story. Sure, you can have subplots but they had better tie in eventually to the main storyline and reach a resolution before the book’s conclusion.
Ansen Dibell says in his book, Plot, that “one of the forces may be external to the main character: a villain, an opponent, a set of circumstances, a feature of the environment or of the landscape. Or both forces may be within the protagonist; the fear of doing something and wrestling with the need to do it; a sense of injury wrestling with love or admiration, as with a person of any age trying to come to terms with a demanding parent.”
A writer’s job is to convince the reader that what's happening in the plot is important not only to the characters involved, but to the writer as well. If the writer doesn’t really care about the characters, the reader definitely won’t.
Struggling to stay alive, such as the young men in Golding’s Lord of the Flies, is the external form of the characters' struggles. But the battle of fear, courage and distrust of the unknown is the internal struggle that has to be addressed to ensure that your story is both gripping and convincing. There should be both an outer and inner plot which mirror or reinforce the other. They may even be in stark contrast, creating an even mightier struggle for the protagonist to overcome.
As one wise veteran writer once observed: "a plot scene should ideally contain two antagonistic characters who duke it out until one scores a knockout at the end of the scene."