Background: Every once in a while, I have a lengthy debate (usually with myself) about the merits and flaws of starting an RPG campaign. I have even put together the world, setting, primary and secondary characters, plot, etc., etc. Basically, all I need is the time and the players, and the game starts. The caveat is that if I start a game, I'm going to have to include my roommate. Normally, I wouldn't have a problem with saying no to this, but since she's also my fiance, there's no way around it.
So, the question at hand (and more abstractly), is how do you run a game when you will be spending a significant amount of time with one player (who will not be able to *not* talk about the game) outside of the normal gaming session compared to the rest of the players, who you will probably only see during game sessions? How do you keep the nice balance of satisfying a player's want/need to talk about the game without actually giving anything away?
You could say you will only talk about the game during sessions or on the computer. She still has an outlet to talk about the game but it isn't quite as easy. Of course, this will only work if she is willing to go along with it. Otherwise it sounds like it is mainly a matter of discipline. I know as a DM in the past I ended up giving up a large part of the plotline earlier than I had planned simply because I was excited about it. The key is to not talk about those things if you don't want to give it away. On the other hand, if a player is really keen, and you don't mind, make it an in character interaction. When she is going on about how something is cool/wierd/etc. the last session was, you could just break in with, 'What does your character think about -fill in the blank- ?' or, 'From your character's perspective.....' I have used this in previous campaigns via email as a way for my more obsessed players to develop their characters and stories further. Of course, this is only if you are up for doing that.
Say, " I'd like to tell you but if I do I'll have to kill your characters".
Jonesin' for some indie action (or WW, or...)
I faced exactly this issue from the other side (me the player, my SO the GM). We did solo scenes that didn't advance the plot but added depth to my character and fleshed out his relationships to NPCs. To be specific---this was a V:tM game---we did scenes with my character's herd, his sire, his contacts, etc. And we didn't do them very often.
GM / Player of many game systems.
To be honest, this is a only a problem if you let it be a problem. I'm assuming that both you and your fiance are adults, thus why not simply tell her upfront what your concerns are and then afterward, only tell things you're comfortable answering. If she asks about something you don't want to divulge, just tell her this info isn't available to players yet or that it would ruin the storyline to reveal it early, or whatever the case actually is. All I'm suggesting is that you treat her like a responsible adult and then expect her to respond like one. If this method won't work, then you have more problems with your fiance than the fact that she's one of your players...
FYI, I game with my wife and have done so for years, so I'm not just talking out of my @$$. The few times she asked me questions outside the game that I didn't want to answer I just told her why I couldn't answer it. She was disappointed to be sure, but she respected my decision.