I'll aside to your aside!
I've been GMing like this for literally as long as I can remember. I've never run APs or modules. The very first thing I knew about my preferences as a GM was "I want to be in on the wild-eyed wonder (as much as possible) of what emerges from play, same as my players!" Then I thought "how do I accomplish this?" Prep what is only utterly necessary for a session (shockingly little to many GMs who have played in my games in real life). Develop a few guiding principles, techniques, and mental-overhead-management strategies which center around how to best evolve post-resolution fiction (to maintain continuity, genre constrains, pacing, and dynamism). Develop the in-situ clerical side (high utility short-hand and make flash-cards my friend). Mature to mastering adlib/improv skills.
When I first read Dogs in the Vineyard, my mind said "holy crap!...this is it!" Then I read Apocalypse World and finally Dungeon World. Then my mind said "holy crap!...this is it-ER!"
I use this technique with some games: Teenagers from Outer Space pretty much only runs this way. Other game styles like my current Conspiracy-X procedural are more focused on the players exploring the secret world and more needs to be known in advance to keep their glimpses of the jigsaw puzzle appropriate. There I generate the initial scene completely and improve secondary scenes and locations that mechanically and plausibly extend from the well understood start.