In any work of fiction, the main character's (or characters') desire drives the piece forward. Don't know what your main character wants? Or can't make that want clear, compelling, and believable to the reader? Then your fiction will never take off.
Desire is linked to motivation: what you want, how you're going to get it, and why you want it so bad.
Because of the important of desire in driving fiction, a writer should spend lots of time understanding and deepening this aspect of the story or novel. This is time "off the page," as I like to say, by which I mean that a lot of what you will work on in developing is for your own understanding of the character and not to be included in the finished writing. Well, it will be included but not word by word... your understanding will inform your writing.
To give you an example of this, I'll use my forthcoming novel, Revolutionary. Early in the story, the main character, Deborah Sampson, runs away from home. Why? That isn't an easy decision to make in anyone's life, at any time, but particularly for a young, unmarried woman in 1782, this would have been an earth-shattering choice. So I needed to understand her motive well.
Given that Deborah is a historical personage, I could look at material in which she discussed (or others discussed) her motive. From these sources, I gleaned that her motive was money (the town gave a bounty for soldiers signing on), patriotism, and freedom. Of these three, money seemed the least interesting: once she had the bounty, she still ran away, so that couldn't have been the sole motive. Patriotism was a nice thought -- and I don't doubt that she was patriotic -- but this was a reason she offered to her later biographer (Herman Mann) and a very convenient reason it was. It allowed her a virtuous basis for an unvirtuous act.
That left me with freedom. She desired to break away from the constraints of society, the bounds of her (medium-sized) town, and, particularly, the limited sphere of being a woman. I knew that's what I had to convey on the page, what I had to convince my readers of. But in order to be convincing, I had to know my subject and context much more specifically than that.
In my next blog post, I'll provide more of the historical context of Deborah, but I wanted to open with this general craft point: start with characters' desires... and let that drive your fiction!