Maybe this is clear in the book and I just zoned over part of it on my read-through.  We started play with just a couple hours available and generated characters and the MC asked some questions.  And in the intervening time, we've done some fiction-building on our local forum.  And there is some concern on the part of the MC that we (players) are taking a too-active role in fiction-generation.  (I hope that's a fair way to paraphrase concerns expressed over several bits of dialog.)

So, it's the MC's job to make Apocalypse World seem real.  And to barf forth apocalyptica.  What happens when a player barfs forth?

What if I, as a player, just say that I'm going to the old air-strip to salvage old engine parts, but the air-strip has never been mentioned before and bother's someone's vision of the setting?  What if it's the MC and what if it's not?

'm not sure what to do when the motivations are really simple.  As in, "I want that bastard NPC to die/fall down hard/bleed."

Is this a good time for an intermittent reward?  "OK, her head caves in like an over-ripe blor-fruit.  She's dead."  Of course, she might have had friends who require some going aggro or seizing by force.