When it comes right down to it the most nefarious and dastardly villain your game can have will be the one you allow a player to play. Why? Well, for one thing the player is going to be going for the win just as hard as any other PC. For another, there is no implied contract - for at least some type of games - stating that the players need to have a fair chance to win or overcome the trials and obstacles before them when those very trials and obstacles have been set there by another player. Rule books and character sheets are full of things to protect the players from the GM, but there is very little - aside from a GM - that is there to protect a player from another player. It is why PvP conflict is such a hot button topic in so many groups. Today though, I want to talk about a different kind of PvP conflict and how it can help enliven a game.
A Cautionary Note
Even the most indirect form of PvP has the potential - at any point in time - to become direct PvP conflict. If your group dislikes PvP or you feel they wouldn't be able to handle it (especially true if you have a player that "has to win" in the game) than this may not be for you or your game. Still, there is some things in here you may like or may just want to try in the future. Just be sure to let your players know if you are introducing the potential for PvP into your game.
If I am going to have PvP in my game I prefer it to be, or at least start, indirectly. How does indirect work? Well, essentially, it means that the PCs are in some form of conflict with each other, but they aren't directly or openly hostile with each other. It is the difference between three different groups wanting a particular object and one of the PCs being a clandestine operative for the fourth group and all the PCs having guns aimed at each other about to pull the trigger until all their weapons do is click.
Now, like I said, Indirect PvP has a tendency of becoming Direct PVP - usually the second one of the combat PCs finds out that another player is working against them. The most fun still happens - in my opinion - when the PCs don't necessarily know who is against them to bring it into direct confrontation. How do you do this? Well, here are some methods.
I'm sure these go buy numerous other names, but at my table we call them Ninja Notes. A ninja note is any note that other players are not allowed to read that goes to the GM. It is just a way of saying what you are doing, what you are up to, or what you go for. A ninja note of "I OD the guy on the knockout serum" can let the GM know you're killing a prisoner before they can talk.
Now, the fun part with ninja notes is that no one knows what they say but generally everyone knows that something is being done. You can have a lot of fun with this by passing notes that say nothing, or just silly things like "look at Sarah, grin, and then write something down before passing back to me." Oh, mind games.
The Other Room
Since time immemorial GMs have taken players to "the other room" for a private scene or conversation. It is a way for something private to happen - something the other players are not allowed to know right now - without anyone over hearing it. Good groups will talk louder while this happens to help cover things. Some groups will suddenly find lots of reasons to move near the "other room" on their way to the bathroom, getting a soda, or just to stretch out their legs.
Ultimately, this has the same problem as Ninja Notes. Everyone knows something is going on. Also, if youre only pulling one player aside on occasion than they also know who it is that is up to shady business. Mind games are just as useful here though. Such as pulling a player aside for a 5 minute conversation about the recent football match and then paying them 5 XP to come back acting like you just killed their character's mom.
Out Of Game Contact
My favorite, and the one I use a lot even in games without PvP is out of game contact. This has all the benefits of a personal touch and private goals, but the best part is there is no way for other players to know something is going on unless someone tells them. However, on the down side, it means that stuff has to happen outside of the normal game time which can make things hard if events are going quickly at the game table.
To solve this I usually go for planning. Try to have time between "the PCs plan their big heist" and "the PCs execute their heist." This gives timefor out of game contact - as well as specific planning now that you know where the PCs are focused - and can also let the player give you a list of contingencies. It may not be enough for "I'm going to try to take the diamond." They may need a list of outcomes and priorities they can act from, and as the GM if you help them with that then you can communicate a lot with just a simple ninja note or other covert gesture.