For me, plot ideas are usually very short, usually in the form of a question (often a "what if" question) -- and they never let go of me until I've written a proposed answer in the form of a short story, a novel, or a poem. Poems are hardest for me to do, and then short stories; novels are the easiest, always. I can write a novel in a year fairly easily, if I'm not too swamped with other work, but it often takes me much longer than that to get a short story into a state that strikes me as adequate. It took me ten whole years to write "Lest Levitation Come Upon Us," which was a proposed answer to the question "Suppose you became a saint accidentally; what would happen?"
The Ozark Trilogy started as an answer to a slightly different question, one that I had posed to my science fiction novel writing class at San Diego State. I had given the students a weekend assignment: "There are all these sf novels where people ride around on flying dragons and flying horses... Find out what Earth creature it would actually be possible for people to do that with." They came back with the answer I had expected -- which is that no such creature exists -- and so I said, "Well, in that case you could pick anything you want .... turtles, cattle, anything. What would you pick?" Somebody said, "Mules!" and I went to the blackboard and wrote "I should have known that something was very wrong when the Mules started flying erratically." And that sentence became the first sentence in Twelve Fair Kingdoms, first book in the Ozark Trilogy. (Which is in print, by the way; it came out in a reprint edition in 2000, from the University of Arkansas Press; there's a homepage for it at my website ...
http://www.sfwa.org/members/elgin ... including a paper doll Responsible of Brightwater, and clothes for her, drawn by artist Karen Jollie. You just click on "site map" and that takes you to all the homepages.)
A few years ago the question that hit me was "Suppose an angel told you to do something and you said no; what would happen?" I wrote the first draft of that novel, polished the sample chapters for the proposal, read a chapter aloud at a science fiction convention to see if people liked it (they did), and turned it over to my agent, who sent it around the markets; it got lots of positive responses, but no sf publisher was willing to take a chance on it. The genre called "Christian fiction" is one of the hottest parts of publishing at the moment (think of the Left Behind books), so I rewrote it for that genre and sent it around the Christian publishers. With the same result; lots of positive feedback, but "too risky for our list." So I rewrote it as a short story -- much harder than writing it as a novel, in either version -- and have been sending it to the small list of sf short story markets. And the same thing is happening. The rejection letters say "I really enjoyed this story, but it's not right for us." I suppose that novel may eventually end up as a poem, in which case it would have been a lot more efficient for me to skip the three previous versions and do it as a poem in the first place -- but I didn't know.
And then there's the question "Suppose we lived in a culture where the most preferred sensory system was touch, and the sensory system that everybody was prejudiced against was sight; what would that be like?" I've read chapters from that novel at conventions several times, and people seem to like it very much -- they especially like the idea that "motherlooker" would be the worst obscenity anybody could say or write -- but sf publishers have without exception said "Nicely written, interesting idea, but nobody would want to read a book about a touch dominant United States; we'll pass." I haven't tried doing that one as a short story yet, because I'm so fascinated by the exploration of that society. (Think of all the questions.... In a touch dominant U.S., what are restaurants like? What are computers like? What are hospitals like? How do people greet each other? What are weddings like? What is clothing like?) I can play with that endlessly, myself.... but it looks as if that may be a personal idiosyncracy.) I may be answering the wrong question....
I don't know where the questions come from; I just do my best to answer them.
Suzette