Trust is, without a doubt in my mind, the most important part of any relationship. Or, at least, that is the case if you want the relationship to work. Because of this it is also one of the most important things you can have in your RPG group. I talk about trust a lot on this blog, mostly about the need for it. Today though I want to talk a bit more about what trust is, why you need it, and why it is very important to not break that trust once you've established it.
Honestly, I'm not so sure I can define trust. In a large way, at least to me, trust is faith, though not necessarily in the religious sense. Faith in a person. This is more that when they say something to you you can believe it (that's just being honest) but more that the person isn't going to betray you. For table top games it means knowing, not just thinking but actually knowing, that what happens in character or in game is only in character or in game. That the real world and annoyances or nuisances that happen in the real world are not a part of it. It is also something that, generally, has to be earned over time.
How do you earn it over time? Mostly by not being a jerk about things. This means early on you may have to play things safe. Don't necessarily give things away with your plans, but you can still let people know. A simple "come prepared for a rough session" is sometimes all you need. Besides that, have in game reasons for everything that's going on and be prepared to share some of it. If someone is going to be betrayed, after the betrayal is resolved, let them know what happened and why to lead to the betrayal. It may feel like hand holding, but it helps build trust and that makes the game worth while.
Why do you want trust? Because with it your game can become more fun, and more awesome. When the players trust each other they can act together, apart, or even against each other without a hitch. The understanding that things are in character only and that people are trying to tell a story lets them check the Out Of Character doubts at the door and focus on the game itself.
Without trust the opposite can become true. Any motion from a player against another player will be looked on with suspicion. Even innocuous actions, such as one PC deciding to also share in on an experience, could be seen as the player trying to muscle in on everyone else's moment. It gets even worse when the players have to act opposed to each other. Ever see a PvP fight when the players don't trust each other? It gets ugly really fast, and the drama can be incredible (not in a good way either) to have to try to handle.
All this means nothing about when the lack of trust is between the player(s) and the GM. With this everything that goes even a little ways wrong is personal or because the GM is trying to protect their plot. The player then either becomes combative, or even worse they roll over and effectively play dead. They don't add to the game and the resentment just grows and grows until, potentially, even the friendship that was there out of the game can be ruined.
So, how do you betray trust and end up in that situation? Mostly, you don't listen to your players or each other. Communication can be a key aspect here because it lets everyone tell the other what is going on. It also gives a chance to simply assert "you know this is only in game, right?" which, sometimes, is all you need. As the GM you need to talk to your players. Make sure they're having fun. Make sure they're ok with what's going on. Most importantly, make sure they don't have any lines that you're near crossing. If they do establish boundaries then do not cross them.
Aside from that. It's fairly straight forward. Run your game. Have fun. Make sure everyone else is too.