I don't need so much as to picture how my characters look like--I'd rather get their personality through first and have readers fill in all the details after I've provided a sketchy physical outline--but I definitely need to know where my characters are. It's a spatial kind of thing: I really don't like having everything happen in a vacuum.
Maps are my friend. With the exception of my first Nano novel where I pretty much wrote everything by the seat of my pants, I've always included in my pre-planning some sort of map, diagram, or house plan. The exact placement of the furniture is a little too detailed for my purposes, but no street goes unnamed.
I'm still not quite sure where I want to fit everything on the island where my story is taking place--but my initial sketch of it isn't much to look at. It resembles a hole-y slab of cheese more than anything else. Which pretty much tells me one thing: I need more research on island maps.
So for those of you still in the initial planning stages--what sorts of tools do you use to visualize your setting? Or are you just going to let description of location fly to the four winds and concentrate on something more character-driven?
