I've been dealing with this with my wife for a little while, so I'll spell out some of my guidelines. For what its worth, my wife is a first time player.

1) I don't talk to her about upcoming plot points. At least for the main plot. Once I realized everyone else had some sort of subplot, I realized she would need one. I talked to her about it, to see what she'd like done, as opposed to everyone else where I made their subplots for them based on their backstory. She didn't really have one, so we needed to hammer things out. As for the main plot, or anyone else's subplot, I do not bring that up with her at all, except to ask for feedback based on the previous session, since I get more out of her than "it was good" which is about what I get out of the other players.

2) Since she's a new player, she needs help with character creation and maintenance (levelling up, spells prepared, that sort of thing). When helping her with this, I have to completely separate myself from the DM's chair and forget everything I have planned. I have been giving her more leeway that I would with any experienced player, but thats more of a new player thing than her being my wife. But since she's close to me, she gets more help from me than any new player probably would (unless they were roommates). IF your fiancee has played before, this probably won't affect you. I have given her a slight boon with working with her character. 3.5 game, she plays a druid, and partway through the game I let her switch from fixed list casting to spontaneous casting, to see if she'd like it more. Since its a power down for a druid, I didn't see a ton of issue with it. I also let her have a folding boat cause she really wanted one, and it wasn't likely to come back and bite me in the a$$.

3) During the game, I'm not really her husband anymore, I'm her DM, and she's not really my wife, she's my player. Her character might die, and I need her to not hold that against me. She acknowledges this, and so far its been smooth sailing.

4) Since I'm trying to not favor her, and I know what her character can do, its actually been working against the character. I've ended up throwing more stuff at them that I know won't favor her, and thats been unfair to her. This is my problem that I've been working to rectify, by for example the subplot above. Kind of the opposite problem as would be the expected normal problem.