Fly or Think? How do you write?

I attended an authors panel a few weeks ago and one of the questions an audience member asked was, "How do you write? Do you write by the fly or do you sit and think?" Even though I wasn't on the panel, I so wanted to answer that question! And now, in a way, I can answer it. It depends on what genre of fiction I'm writing. If I'm writing something like a modern day mystery novel, then I write by the fly. I don't really have to do in depth research and I can write what comes to mind. However, if I'm working on an historical fiction novel, even if it has a mystery thread weaving through it, I have to really think about what I'm going to write. There's so much research involved and I have to make sure the character's dialogue is in accordance with the time period he/she is living in. Even though it can be a challenge to sit and think about what I have to write, versus just sitting down and writing without having to pause and ponder, I like the challenge of doing it either way. So how about you? How do you like to write and why? Does it help keep your creative juices flowing?