[Shane Granger]

I have a campaign going and I was planning on incorporating a sort of double agent NPC into the story, the players are working corporate security and I want this agent to be feeding information to a rival corporation and sabotage the company as much as possible. I have it worked out story wise I'm just not sure what to do with the game play, I don't want to be making dice rolls every time the agent talks to the players since it will make it pretty obvious that shes up to something.

So how do i avoid any plot spoiling rolls while still giving the PCs a chance to catch on.

[Walks Through Walls]

There are a few different ways I can think to do it and a combination of all of them might work best.

1) Preroll the appropriate pools ahead of time and when the situation comes up just look at your notes of what the roll was mark it off and do what the roll results in.
2) Have the characters roll for perception at other times when nothing is going on. The reply with "Everything appears normal." I have found this brings great paranoia to my group  and they get desensitized to the rolls that do matter.
3) Mainly roleplay the interactions, and only have rolls when the character's think something is up. Most GMs I have played with only use judge intent when the character says I'm trying to judge intent then the roll is made.

Hope all of this helps

[Shane Granger]

I love your second option I'm definitely going to do that, even after this double agent thing is over. that combined with number one completely solved my problem, thanks a bunch.

[Walks Through Walls]

Your welcome.
I'm glad I was able to help you out.

[Leticron]

One of my players once decided to play a double agent himself and i would handle any NPC the same way, meaning, whenever you absolutely have to make a dice roll, just also roll for the NPC (or let the Player roll) so that your (other) players don't get suspicious of him or her.
Apart from that I'd strongly suggest to limit dice rolls to a minimum in the actual situation. "walks through walls" has already given you some handy advice.

Generally speaking, though, you should create a way of gaming in which dice rolls do not automatically indicate that "something is happening" or players just failed to notice that "something that is happening just now but you don't have any clue about". Personally, i tend to reward well placed perception or insight tests as i usually drop some hints that players can catch. Should they get all paranoid however and try to overdo that, I feed them misleading "clues" in their growing paranoia. That combination has played out quite well so far.

Hope i could help.