There are a few ways where I get a story idea. One is by taking an existing idea and twisting it somehow. Another way is a character idea pops into my head, mostly fleshed out already and needing tweaking. A third is looking at a genre and working out a general story idea that fits or plays with the basics.
Working backwards on the above list,
The genre story.
Typically, I'll look at the genre first, then fill in the needed roles with characters. The MSTing crew back in my Shadowrun intro began this way. A typical group of runners in the game needs a magician, a hacker, some muscle, and a getaway driver. I don't state it outright in the story - I was using it to establish characterization for a larger story - but, hopefully, it could be inferred that Treehugger is the getaway driver, that Charles is the muscle, that Numbers is the hacker, and that Oswald is the magician. Other stories I've worked on were similar; a group of magical girls, with personalities painted with broad strokes at first, then nuances brought in. The central magical girl was friendly, easy to get along with, empathic, everything needed to pull her team together. Her younger sister was more impatient, not willing to sit still when something happened. The best friend was a shy athlete. The reluctant magical girl was an ambitious workaholic, whose rival was flirtatious. The sixth magical girl was the opposite of the workaholic; fun-loving and easily distractable. The Traveller story I'm working on has ship-based needs to fill - a pilot, an astrogator, a gunner, an engineer.
However, just filling the role isn't enough. The characters do need to feel real when the story gets read. The first chapter needs to establish the characters, but they need development. Thus, with the Shadowrun group, I wrote the MSTing, with each person in character somehow, whether it was in the interview or in commentary. Numbers managed to present a professional side and a personal side she shares with her comrades while Oswald displayed cynicism and a sardonic wit. With the magical girls, the broad strokes needed nuance. Each girl got a family, a back story, that either explained her behaviour or gave her something to rebel against. With the Traveller crew, I will need to write out some vignettes to flesh things out before I tackle the main story.
With a character-based story, the characters come to mind first. Usually, the story revolves around one central character, though a small group of two or three are possible. In this case, the character is mostly formed in my head already. Small details, like setting and plot, can be coaxed from the characters. A previous NaNoWriMo work started with a pair of characters, Bronya and Morwenna, who were supposed to be in a medieval fantasy until they complained, then showed me in a chase involving flying cars, magic spells, and Bronya getting ready to leap from one car to another while holding on to a katana.
The medieval part got dropped.
Granted, this is an unusual method of creating a character. It probably flies in the face of the usual advice given to writers. But, it works. Mostly. Sure, their story isn't completed, but that's because I didn't work out the entire plot. It is not because I didn't know what Bronya and Morwenna would do. This was a case where, no matter what I gave them, they'd know what they'd do, even if it was to tell me that they didn't like the idea. Their supporting cast grew out from them as I added details of family, of past lovers, of types of magic. Whatever happened in the plot, though, I knew what the reaction would be and why, because of how well-formed they were when they approached me.
Twisting an Existing Idea
Creating characters for stories that come from exploring an existing idea and what previous works missed is similar to the genre work, above. Roles already exist and just need populating; typically, the role already has a personality type defined for it. All I need to do is drop in a character and tweak as needed. With the twist, though, sometimes the pre-existing personality type needs to be altered. If, in a space opera, I have a brash smuggler with a technically inclined co-pilot, I might change the brashness to something more sober. The co-pilot may pick up some of the impulsiveness to drive the plot, but not the same degree as the smuggler's original archetype.
One idea I have, which will be more fully fleshed out in a future NaNo Prep 2013 entry, is a spy thriller crossed with urban fantasy. It's a two-for-one twist. First, the spy thriller, along the lines of James Bond and Jason Bourne. A lone spy saving Western civilization from the predations of the villain-du-jour. In the 60s, the villains were the Soviets. Today, it's a toss up between terrorists, hackers, organzied crime, or a mobster hacker funding terrorists. The second twist is on the urban fantasy side. Urban fantasy covers a wide range, from Jim Butcher's film noir Dresden Files to Jennifer Estep's supernatural assassin to Linda Poitevan's police procedural to Joss Whedon's high school vampire slayer. The setting is the modern world, with the supernatural lurking out of sight, much like the classic spies in fiction. I know I'll need three characters already, my spy lead, most likely British or American, my supernatural lead, and my supernatural villain. The spy is the most critical, being the character that readers will follow into the supernatural world. The catch now is to play with expectations. Will the spy be a spiritual descendant of Bond and Bourne? Will there be Q-style gadgets and vehicles, or do I go in a more realistic manner? Right now, the idea is for a more down-to-earth approach, with the agent having backup as seen in the hunt for Jason Bourne in the Bourne movies, but with some flair tossed in. Meanwhile, the supernatural lead is Jack, because, in his words, "Everyone trusts a Jack."
I don't trust him already.
Coming soon, naming characters.