Watching Diana write
Here's a little video demonstrating how Diana Gabaldon writes.
I saw Diana do this same "watch me write in 90 seconds" bit in person once, at an appearance a few years ago. But I really enjoyed the animation in this video, because it's easy to imagine Diana moving words or phrases around on her computer screen in exactly that way. "Fiddling", as she calls it.
Imagine 800 pages full of sentences constructed with that much careful attention to detail, and you begin to see, as Diana says, "This is why it takes me a long time to write a book."