GMC stands for Goal, Motivation and Conflict. To get the real scoop on GMC you need to refer to Debra Dixon. She literally wrote the book on Goals, Motivation, And Conflict and does lectures on it for Romance Writer’s of America both in the RWA Chapters and at National.
The point of the book and lectures is that every effective character comes equipped with at least one set of goals which are motivated by the character’s beliefs and that the goals are thwarted by something, thus generating conflict. She goes on to discuss internal and external goals and how to set up a chart showing how the goals, motivation, and conflict work together to make a GMC matrix. (Note: I think she used the word matrix for it. It’s been a while since I read the book and I can’t find my copy. If she doesn’t say “matrix” to mean the way the parts of GMC come together then tough because I’m going to.)
First, like most tools for working with the structure of fiction, focusing on GMC while working on the rough draft can mess you up. I have found it useful in plotting before writing and extremely useful in both revision and critique, and am now so comfortable with the concepts that I can even think about them while producing dialogue and action narrative. But trying to keep goals and motives and conflicts in mind while putting the words on paper before you are really comfortable with GMC can give you writer’s block. So if this is new to you, don’t worry about it while you are actually writing.
Second, Debra may say that every character needs his or her own GMC, even the waiter delivering an order. Personally, I haven’t the time. I definitely recommend having a clear GMC matrix for the protagonists, and maybe for some of the more prominent secondary characters, but face it, some characters are little more than furniture. Let them fulfill their roll and don’t worry about it.
Now lets talk a little about why you want to worry about GMC at all. I know a number of people who fight the idea of putting conflict in their work, or dislike working with a character who has goals, or who aren’t comfortable with motivation. You want a good combination of GMC because a really good GMC matrix gives your work what is kindly referred to as “good suction.” In other words, the story sucks the reader in, then draws them helplessly through the entire story until the very end when they should “That was great! Let’s do it again!” You want that, right?
It’s the way a character reveals him or her self through the struggle with the central conflict that keeps us turning pages. Finding a goal and motivation that apply to the central conflict act like the strings on a marionette – when drawn tight the jumble of parts turns into something approaching human.