I'm just blocking in the rough layout of everything for now. Once the core game is built, I'll go back and swap things out and make it look more realistic/interesting and have more items you can interact with. There will be Rift style puzzle and things you have to do, not just a scary walking sim.
I spent the summer planning out the main plot and the entire level structure for about 100 different 'areas' (although some are smaller or you can skip them depending on the route you take).