In MISERY, Stephen King's writer protagonist makes a distinction between "having an idea" and "getting an idea." In the novel the writer's "number one fan," enraged that he's killed off her favorite character, demands that he bring the character back to life—but she must be resurrected plausibly. The author has an idea: His heroine was mistakenly buried alive. Now he needs to "get an idea"—to build on the moment of inspiration by figuring out what put her into a death-like state. C. S. Lewis made a similar distinction between the story concepts that spontaneously flash into a writer's mind and the linking material that must be consciously invented. He said ideas usually came to him in the form of mental pictures (e.g., the Narnia series began with an image of a faun carrying an umbrella and an armful of packages in a snowy forest). When he had received a number of "pictures" that seemed to belong to the same story, he then had to do the deliberate work of constructing a plot to fill in the gaps.
My experience is similar, except that I don't have a very visual imagination. I "have ideas" in the form of concepts, sometimes with a protagonist and scraps of dialogue attached. Right now I'm trying to compose a paranormal romance novelette for the new Silhouette Nocturne Bites line of electronically published stories. Immediately I knew I wanted to set my story in a Lovecraftian background. The first idea that occurred to me was a young woman who possesses an artifact that allows the wearer to control certain creatures from an alien dimension (I imagined them as similar to the Hounds of Tindalos in the classic Frank Belknap Long tale). The villain will try to steal the artifact and will succeed in stealing another object she owns, one of those forbidden books ubiquitous in Lovecraftian horror. The heroine and hero will fall in love while tracking the villain. Then I had to start working out the details. Since I want the heroine to be ignorant of her potential power, I had to explain why she owns these objects in the first place. I decided she inherited them from her late uncle, an anthropology professor. She should also have untrained psychic power of some kind. The hero should also have power, but he would know the secret behind it and the artifacts. How did he get involved with her? Silhouette Nocturne wants "alpha" type heroes, so if the hero was an archaeologist who worked with her uncle, he'd have an adventurous, somewhat exotic background and personality to fit that requirement. Also, this back story explains how the heroine could be previously acquainted with and secretly attracted to him. Moreover, I decided their ancestors came from the same village many generations back, as an explanation for why they have similar powers and why the hero sought out her uncle as a mentor to begin with. What kinds of psychic talents should they have? I considered and discarded several possibilities before settling on telekinesis, a useful all-purpose gift. I also decided they would have an empathic and erotic bond activated when they both touch the artifact simultaneously. What kind of artifact? Rings and amulets (necklaces) are overused, yet it has to be something that can be conveniently worn by a person of either sex. I thought of a bracelet or armband, which sparked the image of a bronze armlet etched with arcane symbols, an object that could be of great antiquity.
Because I'm an outliner, not a pantser, prone to paralyzing anxiety if I face a blank screen with no idea of what happens next, I need to work out these details and many others before I start. My problem with brainstorming is to keep myself open to alternate ideas as far into the process as possible. I'm too inclined to let myself get locked into one particular sequence of events too early in the planning stage, thereby missing other fruitful plot potentialities.