As in so many things, some of both, though more the former. I started out always working from the seat of my pants, but got tired of getting stuck on middles. Plotting ahead keeps me from getting into those dead "stuck" areas, for the most part. But even when I start with a plot in mind, there's so many things that are just potential, that aren't real, until the words area actually on the page.
2. Detailed character sketches or “their character will be revealed to me as I write”?
I sometimes do character background stuff, but it isn't real until I'm writing. I think it comes from my gaming background, where I designed a set of characters to go with a scenario, and then got to watch different people play the same exact character--well, the character was the same on paper, and the broad description never changed, but they were SO very different in the reality of the action. Amazingly so.
3. Do you know your characters’ goals, motivations, and conflicts before you start writing or is that something else you discover only after you start writing?
Mostly, I discover things as I write. Though I have to admit that the dreaming/thinking/imagining part is as much a real and necessary part of writing as the typing part. But everything can be changed before it's typed into the story. No--wait--it can be changed even then, but there's more work involved.
4. Books on plotting – useful or harmful?
Sometimes useful, but in a limited way. The gestalt of the story--the characters and world and action and mischances and all--is a much better governing factor than somebody else's theories of writing. And the next most important thing to keep in mind is the reader, giving the reader the parts of the action that are needed for understanding and for the enjoyment of the story. Every rule that exists can be broken, and sometimes must be broken to get the best story possible out of the raw materials on hand.
5. Are you a procrastinator or does the itch to write keep at you until you sit down and work?
Again, both. When I know what I want to say, I feel a strong need to get it down on paper, before it mutates or vanishes. I may still change it later, but I don't want to have to retrieve it after my brain moves on to the next plot problem or whatever. But when I don't know what I want to say, giving my brain time to process is good. Though I'm not sure it's "procrastination", which is putting something off until later that you could be doing now. If the idea is still brewing, then I can't write the same thing now that I could write later.
6. Do you write in short bursts of creative energy, or can you sit down and write for hours at a time?
I would guess that whoever wrote these questions didn't imagine they would get so many "both" answers.
7. Are you a morning or afternoon writer?
Evening and late night, mostly. But any time is the right time, if I know what I want to say and have time to say/type it.
8. Do you write with music/the noise of children/in a cafe or other public setting, or do you need complete silence to concentrate?
Music is good; silence is good; company is good; a guardian cat is good; sometimes even TV is good.
Bite your tongue! Longhand is distressingly slow, and I hate typewriters. Though you use what you have, when you don't have the right tools. But computer. Definitely computer. And an ergonomic keyboard if at all possible.
10. Do you know the ending before you type Chapter One?
Only in general terms, and not always. And even if I know, as characters actually do things, my understanding of them and the setting and everything else internal to the story changes and grows, so the ending I "know" will happen as I write some early chapter is often not the ending that actually happens.
11. Does what’s selling in the market influence how and what you write?
Depends. I have been thinking about what short story to write with a myth or folklore theme for several weeks now; the idea that finally stuck wasn't the first one I considered. If someone has several ideas that are equally compelling in the writer's imagination, it's perfectly reasonable to pick the one that's most likely to sell to work on first. Likewise, if the theme of an anthology or the like sparks an idea, that's a pretty cool thing. However, a religious inspirational football story is right out. At least, unless you're paying me a WHOLE lot.
12. Editing – love it or hate it?
Editing is good. If you're careful who you let do the editing, anyway.