The controlling idea is your story's meaning. This is what your readers or audience will say, when someone asks them, "What's it about?" This is the foundation of your story.
To discover your story's meaning, you should know in some way how your story will end because your story tells the idea, not you. The controlling idea is the change in value that happens to your protagonist as a result of the climax - value plus change; the how and why of the change.
The controlling ideas is important because it is the vision of your story that the readers will remember and will be persuaded to believe it and live their lives by it. The idea gives us meaning and tells us, 'life is like that.' You must believe in your controlling idea. If you don't believe it, then do not write it. This is also where you should question what you want to say visibly and concretely through dramatization, not explanation.
Some analysts often also call this the premise or dramatic issue of the story so if you run into it don't be confused.
Value: The dominating positive or negative value that forces change. Values come from human needs. Examples include love of God, love of man (not only romantic love), justice, happiness, self-esteem, growth, freedom, etc... We can also explore the negative sides of the values such as hate, indifference, sadness, pessimism, etc...
Cause: The change is the reason why the value changes.
Love endures even when there appears no hope of being together. We see this in Romeo and Juliet. Both characters commit suicide because of their true love; even their death won't impede their ability to love each other.
In Misery, Stephen King used the word captive in the premise of his story. When we're held captive, we lose our freedom. At the climax of the story in the movie, the protagonist, Paul Sheldon, gains his freedom when he fights for it. For the controlling idea, we can say: Freedom is attained when it's pursued. This theme (another word for 'controlling idea') is found throughout the book as we shall see in the following articles.
Tape your controlling idea to your computer, to your notebook, everything you write should be filtered by the controlling idea. If it has nothing to do with the controlling idea, don't put it in your story.