“At the beginning of every season, we sit down – maybe eight or nine of us – in a room,” says Evan Katz. “We talk for about two hours a day for weeks and weeks. It’s draining, but what emerges are ideas about story construction, and villains, and settings. We leave that room with a good idea about the first four episodes.”
Evan Katz has been a writer on the show since series two, in this interview he offers some frank answers about the writing process on the show and it’s future.
source : news.independent.co.uk