If you only plan to sketch out your outline before you jump into writing your story, you’re ready to go. If you want to know more, it’s time to flesh out your sketch. In order to do so, you need to gather more insights about your characters and your plot, and work those into your story.
I find it helpful to have a set of questions you ask yourself with every novel-length story. These questions should cover elements of your characters, their problems, and the plot that flows from them. For example, what makes your protagonist compelling? Why would a reader stick with this character—root for (or in some cases, against) this character—for the length of a novel? What will make your plot compelling? What kind of sub-plots will you have?
Currently, I’m using a series of blog posts written by John D. Brown at SFWA to trigger my questions. Brown’s posts are thorough and informative. I supplement these posts with lessons I’ve learned from other sources, but this is the guide I’ve been following as I brainstorm my two current novels-in-progress.
Use your sketch like a skeleton and flesh it out with twists, turns, and revelations. Ask yourself questions, answering those questions, and recording what you’ve learned. I usually end this process with about 15-30 pages of bullet points. You may have more or less than that, and you probably won’t include everything you learn in your novel. But it’s a good way to add detail to the big picture you’re trying to create.