So, one of the major points of the worlds is that they're all separated by invisible walls, and that no one knows that there are other worlds out there, save for a select few people. And it must be kept this way to maintain the world order. The thing is, I think after the events of the first Kingdom Hearts, a good chunk of people already know, or can guess, that something is up. Not just because Sora is running around visiting, or the appearance of the Heartless. I'm referring to the fact that the Heartless went on a world eating spree, and when they get to the heart of a world, there's a pretty big laser show, if what happened on Destiny Island's is any hint. So in places we know that got consumed, such as Destiny Islands, the Land of Dragons, the Pride Lands, and goodness knows how many others, a lot of people got to see the fireworks. Not to mention that popping back into existence suddenly probably had some very weird sensations that not too many people are likely to forget. 

 

So I guess I'm just curious to know how people walked that off in the different worlds. 

I don't think the residents of each world knew Heartless came from other worlds. They must've thought Heartless spawned within their own world.  

It's not entirely known if the people who 'popped' back into existence were even aware they were gone in the first place. I always thought they had no memory of the event, much like if they were turned into Heartless. I find it more odd that characters like Mushu and Simba never bothered to mention the universe-spanning adventure they went on with Sora in KHI, but have no problems admitting that they know the guy. Were they sworn to secrecy or something? 

They could've just dismissed it the way Sora and Ventus did: as a meteor shower. Even if they thought some weirdness was involved, there's no real way to connect the phenomenon to the presence of other worlds, unless someone came up to them and said, "You know what weird meteor shower you saw? That was another world exploding. Just to let you know."

They could've just dismissed it the way Sora and Ventus did: as a meteor shower. Even if they thought some weirdness was involved, there's no real way to connect the phenomenon to the presence of other worlds, unless someone came up to them and said, "You know what weird meteor shower you saw? That was another world exploding. Just to let you know."

True. And it's not like anyone's going to educate them on what's really happening anyway, not unless they're keyblade wielders.

I think what Nomura has been suggesting (based upon re-reading past Ultimanias and observing the games in general) is that the people of the worlds, unless they're extraordinary, probably don't have any idea what happened to their world once it's restored. They're probably just aware that a storm happened, but then once the world is gone, they're all asleep. Once the world is saved, it's either stuck in sleep or it's brought back to the Realm of Light. Everybody wakes up and they only remember a huge storm happening, but that's about it. They just seem to dismiss it as strange and go about their daily lives. If anybody was missing or left, they may remember that they left and that's it. Still, this is never stated in-game, you can just sort of assume it from what's given to you. It only appears that the characters who are important in the worlds are the ones who retain any knowledge of outside worlds (normally because they are actually integral to the plot), while everybody else I guess just sort of shrugs it off. Hopefully they might be able to supply a more definite explanation in future games though.