This is going to meander for a bit, so bear with me as I slowly move toward the point.
A long time ago, I played my first game of Pendragon. I had heard about it, of course, but it was my first opportunity to actually play the game. Now, one thing I've never been secretive about, but which I don't bring up unless it's relevant, is that I'm a "pagan", or polytheist. When atheists bring up the argument that "we all don't believe in Zeus, why not just not believe in one more god?", I'm left laughing, because I "believe" in Zeus (as it were - "belief" is a complex subject for me; suffice it to say that I "believe" in Zeus in the same way that someone else might "believe" in the Prime Minister of Japan), among other gods. This is relevant because, when we were making characters, I found out that I could play a "pagan" knight in Pendragon. That was pretty cool to me, since at the time I was still just finding out that there were other polytheists in the modern, Western world, so of course I made a pagan knight as my character.
Here's the thing about people who are coming into an identity, and it was true of me at the time: they tend to be pretty intense. Since they are finding self-definition in terms of that identity, they tend to see everything in terms of that identity and relations to that identity. It's kinda like the old saw that when all you have is a hammer, all problems look like nails. Anyway, at the time, I was pretty strongly involved in my identity as a polytheist or pagan. Another thing about identity is that, when a person is strongly involved in it, people who don't belong to that identity are something of a threat. They either challenge that identity or create a boundary that reinforces that identity. It is pretty easy, actually, to force the challenging factors to conform to the reinforcement role by treating them as opposing forces. That is, someone who is not part of that identity can be either someone who presents an opportunity for deeper thinking about that identity and so deepen our connection to that identity (but this has two drawbacks: first, it is difficult work, and second, it can cause us to reject the identity), or they can be an opposing force that causes us to entrench more strongly in that identity. We can force people from the first category into the second by redefining them as "enemies". It is also possible to force people from the second category into the first, but that requires a subtlety of approach that is very rare (though it can be learned).
So, here I am, playing a pagan knight, with all of this identity matter still running hard in my head (I think I can't have identified as pagan for more than about 5-8 years at that point, and my serious involvement must have been less than 2). The scenario was set at a tournament, at which there were (of course) both Christian and pagan knights. Step by inexorable step, I found myself building a narrative where my pagan knight - and the other pagan knights were following my lead here - was in opposition to the Christian knights.
The thing is, in Pendragon, there isn't a strong theme of pagan vs. Christian. The GM of the game also didn't really want to deal with that theme. So, his first reaction to this development was to tell us that there wasn't a big pagan vs. Christian vibe at the tournament or in the world at large. He quickly backed off of that, to his credit, but that initial reaction, to try to control how the players reacted to the world, was an interesting one in retrospect.
At the time, gaming was starting to move in the direction of "story" that would ultimately result in the White Wolf concentration (in theory, at least) to the extent of redefining the Referee as a Storyteller, and then to the modern "story games" that bear only slight resemblance to the earliest adventure games. As a result, the idea that the GM was presenting a story within which the players would have some interactive power was becoming fashionable. This is a perfectly valid way to play this sort of game, of course, but the reaction of the GM of that Pendragon game is particularly interesting in that he stepped back from imposing his story. His impulse was to ask the players to head back to the story he wanted, but he chose instead, at least ultimately, to allow the players to lead the game where they wanted it to go, for good or ill.
For me, this was a critical moment in my gaming history. I started at that point to see that the "storytellers" in a roleplaying game were the players. The Referee was not the storyteller (though I would be derailed from this train of thought by White Wolf, and later by story games, for a while), who instead resembled something more like an editor.
What that long-ago Pendragon GM should have done, rather than tell us that "there's not really an opposition between pagans and Christians", which was basically telling us what our characters should be thinking, was to either let us run with what we wanted our characters to think (to be fair, that's what he did, in the end) or to, at most, present an NPC authority who would try to guide our characters. For instance, an older Druid priest who gave our knights guidance and tried to smooth over our conflicts of religion by, I don't know, teaching us about the Grail or the way that the Christians' idea of the Crucifixion resembles the native ideas of the King in the Ground (such as the head of Bendigeidvran, buried and protecting the island of Britain from invaders until it was disinterred). It's because not every person in a game world is going to be seeing things the same way. Like in our world, where on the one hand you have people like the Quakers, who are Christian, but also people like Fred Phelps, who is also Christian. Which one of them is correct? Sure, most people would pick the Quakers as better people, but the Phelps family are still Christians. There isn't a GM out there telling them that Christians don't think that way, but there are people (and perhaps spirits, if you accept that sort of thing) who are, and people who are telling them the opposite. It's still down to the Phelps family members, though, as to what they choose. If they did not have those choices, then they would not be responsible for them. Similarly, players should be responsible for the choices they make for their characters, and if the GM is making decisions for them, then why are they playing? (That gets a little more complex when morale and temptation systems come into play, or the personality trait system from Pendragon, but that's a topic for another time.) That's one of the reasons that I think the Alignment system as presented in AD&D is a problem and should be discarded.
Which is me coming back again to the idea of the sandbox, where the players are allowed to have their characters do what they want those characters to do. The environment can react to the characters, and there can even be NPCs with goals who actively affect the environment, but the environment shouldn't be altered for metagame reasons (that is, it should follow the classic idea that, if there are six trolls in a room on the third level of a dungeon, then if a single 1st level character shows up there will be six trolls, and if 20 10th level characters show up there will be six trolls, but if the trolls have been attacked and send for help from the hobgoblins there might be an additional 20 hobgoblins along with the surviving trolls the next time the players show up; I guess that one is a pretty big room).