I only start with an idea, and possibly a few NPCs, and then I let my players actions decide what happens next. Lets say that the idea is that the players are going to rob a grocery store, then I might have penned out the person who hires them & possibly an important person in the store, but the actual store layout is not set in stone. I'll freeform the building as I go along, add people, features and whatever else might fit. Then, depending on my players actions, events will unfold. Lets say that they break in through the back door, but don't check for an alarm, well that might mean that the police will be there in 10min. Or they might be extra careful, and then I might add a guard dog (and a guard) that they will have to find a clever way of dealing with.