It just comes to them. I know when I start writing, I don't usually intent to write something long, but once I write down one thing, I think of a hundred other things that could happen and it just goes from there and before I know it, I have at least 20 pages more than I expected. I know I'm nowhere as great as either of them, but I imagine it would be a similar happening when you just can't end something because there's so much more to be said.
Stephanie Meyer wrote the Twilight series as well as The Host.
