Unless I have something pre-planned (i.e, the bad guys the PCs were expecting to be arriving at dawn instead arrive late due to them meeting somebody or something else along the way), I rarely mess with player plans. I've become adept at tuning them out at the table while they discuss strategy and what to do next in game... I usually do some prep work, organize some miniatures, maybe roll some initiative dice, etc.
The only time I really messed with actual plans in game, I had kind of already set them up for it in previous sessions. One of the players had mentioned a few sessions earlier something like, "now that we've been noticed by followers of the BBEG, we should try to protect ourselves from things like scrying."
They all agreed that they needed to do something about it, both in game and out of game. However, they didn't really get around to actually acquiring the protections or casting the right spells, even though I did remind them of it at least once in passing, and the group had discussed it on the email list we have.
They went through a few more encounters with the bad guys, until they finally knew where one of the BBEG's main lieutenants was having an important meeting with some other followers. So, I let them debate how they would ambush this lieutenant and the other bad guys with him.
However, as Admiral Ackbar would say, "It's a trap!" - the BBEG was scrying on the PCs and knew of their plans, so he ambushed the ambushers, and it ended up being a very tough & memorable fight, though the players managed to eke out a victory.
Needless to say, the group then took precautions to protect themselves from scrying/spying after that.
And, I did give them the satisfaction afterwards of letting them know the protections worked, "you feel a tingling in your head and a force attempts to enter your mind. However, you then feel a warmth coursing over you from the amulet you wear, and your Will easily rejects this feeble attempt to penetrate your head..."