Maybe he's just curious about the difference between what I planned and what really happened during the session. Maybe he wants to get ideas, or maybe he just want to know "how stuff works" behind the curtain.
I love these questions, but I don't usually answer them - at least not right away. I love these questions, because they mean that someone is at least interested enough to ask them.
I don't usually answer them because answering them might say too much about my way of thinking, breaking the player's suspension of disbelief and possibly taking away some of the magic taking place around the gaming table.
But here are some role-playing "secrets" from behind the DM's screen I am willing to share with the world (and all my players):
Having the right players is key to the success of the campaign
You know these guys. They'll do whatever they can to keep the game going. They'll show up to your sessions and will do anything they can to help you and the other players have a great time. They'll come up with ideas for cool things to do, they'll interact with your NPCs (even if you suck in portraying them), they'll bite into your half-baked hooks with a wink and they'll pop a good-hearted joke when you fumble with some plot element. In short - the game just gets better when they're around. They're worth their weight in gold. If you find such players - do whatever you can to keep them around!
Prep, prep, prep, improvisation!
Coming up with a good story to drive the campaign isn't hard. Coming up with an good, original story is. This is why my stories are never original. I steal what I can from movies, TV shows, books, comics, and other DMs. I just make sure to coat everything with my own, special flavored sugar to make it somehow distinct. I do lots of prep (just ask my wife), trying to make sure 99% of what's going on around the table is (at the very least) thought of. But - every now and then things get "out of control", when the players do something unexpected. Suddenly, all my precious DM notes are irrelevant. You know what - it's even more interesting that way. As long as you've got those "right players" in your group, everything will be all-right. The campaign doesn't break because someone does something unexpected, and the more experience you have, the more you become proficient in handling these situations. Let the players tell their own stories - it's your game their playing, but it's not your show.
One common goal, many stories to tell
I learnt that the running the game is easier when the group share a common goal they thought of together without the DM forcing it down their throats. It's not that hard to pull off - it just means that the DM cannot build a complete campaign before the group meets (is that a real issue?). Have the players talk about what they want to do in the game, have them think of a way to form a group around that idea, and you have a bunch of gamers ready to start their first adventure with a specific goal in mind. It's like getting into a car that's already started. Each character should still be distinct, with its own story to tell, but having a common goal really helps to get things going. Then, as the players pursue their common goals, their individual stories can surface every now and then, and so they have a way to "side-step" and refresh themselves when the main goal becomes a bit of a grind.
Listen to feedback, but make your own choices
Every now and then a player will come out and ask for something. Maybe he wants more political intrigue in the game. Maybe he wants a chance to research a magical item. Or maybe he wants things to move faster, as he's bored with all that "talking" taking place in recent adventures.
Now - feedback is always welcome. A player that takes the time to talk to you after the game about something that bugs him is a good thing. You can learn a lot about how your gaming sessions look from the other side of the screen. But make sure to understand that all you get is that player's perspective. If one players asks to have more fights with undead, then OK, you might want to introduce an encounter to satisfy his whims. But if 5 players ask for the more fights with undead - you are onto something BIG. Maybe your players have an itch to scratch, and the story so far don't even come close? Maybe they had some expectations that weren't met? Talking to your players can give you a lot work with. And even if a player says "I'm not happy, you should fix this and that", don't fret. Fixing "this" or "that" might earn you a happier player, and if fixing it doesn't make other players unhappy, then its a win-win situation. Just make sure this is the case before making drastic changes to your campaign.
Rules are important, but not to the point of going to court
I personally believe that a rule-system gives a certain feel to the game, and I prefer running by the books than house-ruling my own flavor of one system or another. I also feel that players operate better when the rules are on the table - things are less arbitrary. You should use a rule-system that you are comfortable with, one that helps you getting through that sense of the campaign setting to the players. But that's it. The rules should help you create and run your game - but it is your game. Don't let the rules (or rules-savvy players) dictate how things work around your table. Be consistent, be clear, and be fair. If you have those "right players" around, you can count on them to help you run it (especially if they know the rules better than you), but they'll never take your position as the final arbitrator, or question you just for the sake of making a point.
Finally, tell an interesting story
It's not hard if you remember that the story is about the player characters, not your NPCs. The story will be interesting if it's about the characters your player run. If a player hands you a background full of details about his character - then you have to use it as a basis for the campaign. Otherwise, you're telling your own story, and it will suck, because the players don't come to your table to participate in your show. They come to put up their own show while playing your game.
It's very hard to trust players to the point of letting them run the show - I know. It doesn't mean you do everything they ask. It means you take into account their stories, likes and dislikes when planning the adventure - even before starting the campaign. You challenge them, making their quests interesting and unexpected, helping them to tell their characters story in your world. Now, for obvious reasons, most players are not invested in the campaign as the DM is. But if you try and have them tell their stories instead of yours, you might be able to create something that is really unique and even beautiful - a unique experience of shared storytelling done around a table with a bunch of friends.
It gets even better with Pizza.