It turns out that published adventures are seldom good examples of the best practices for organic-feeling games as they develop at the table. Unless a published adventure is an absolute railroad, there will likely come a point where the players do something the writers of an adventure just haven't accounted for. The same thing will happen to your carefully-written and fully fleshed-out homebrew adventures. If divergence is nearly inevitable, why fight it?
Prep less, but prep smarter.
One thing I've found extremely helpful is to prep a list of NPCs that the player characters might interact with at some point. If the action ever flags, it gives me a ready-to-go idea of who I can throw at them, who has schemes that might embroil them, and who might seek their aid to get something done.
I keep the list simple; three columns, maximum: name, a short physical description, and a motive. The last one is key. Give each NPC on your list a strong motivation: something they desire above all else, something concrete to base their actions around, something that will color how they interact with the player characters.
I advise you to give each NPC one central motive. It's fine, and maybe even preferable, if your NPCs aren't nuanced and holistically-crafted avatars of verisimilitude; a focused, immediately usable NPC is so much more valuable than a complex, "realistic" one.
As for physical descriptions, just jot down two or three small-but-memorable things; you don't need a paragraph of description or a compendious backstory. And make sure you give each NPC a name because making up a name in the middle of the game can suck.
But what about stats? I don't worry about it until the stats become important. I find that stats are usually secondary, but if you feel you need them ahead of time--stat away.
