Share Focus
Multi-participant games require sharing. That's something that you were probably taught in kindergarten, but it's a thing that is really important when it comes to gaming. What you really need to share in the RPG, whether tabletop or LARP, is focus. This is one of the things I picked up in improv, but basically, the narrative should shift from one character to another, from one player to another, as the story flows. Think of it like a conversation. Boring conversations happen when one person talks all the time and the other person can't get a word in edgewise, sharing focus in game is giving the other person time to speak.
This can be done passively, but it can also be done actively. Some of my favorite players are those who delegate story tasks, taking players who aren't actively engaged in things and putting the focus on them. Sometimes this can be a matter of faction leading, if you are a faction leader, having minions that you send to do things is a good methodology.
Ideally, everyone should get to do something at some point, but part of being a good player is being willing to share focus with your fellow players.
Yes, and...
Another concept from improv that I think makes for a better player is the incorporation of the principle of "Yes, And..." In improv, it's about taking what someone adds to a scene and building upon it. When someone says, "hey, look at that kangaroo" you don't say, "there's no kangaroo" because that sets the scene back to square one. Instead you say, "Yea, and I think he's stealing your car!" accepting what someone gives you and going with it.
This can be very useful because it's part of constructing the game world. Most games that I run have a framework world, one in which the large details are filled out, with the smaller details remaining nebulous until relevant. This is the case for nearly any fictional world. You don't know what color Harry Dresden's socks are until it matters for the story or characterization purposes. Filling in these details and building upon them can be an excellent thing for players to do in game.
Some Storytellers don't like when players fill things in, and that's a matter of taste, but I'm happy to have players drop in details like frequented restaurants with revolutionary themes, foul mouthed baby goats, or past experiences. By working together to fill in and following the principle of "Yes, And..." a player can help build the world around.
Engage with the Content
That said, one of my greatest frustrations in a game is when there is well developed content that is left unengaged with. So, as I said, I like to develop a framework world, one in which there are several overarching rules of reality, aspects of the world that are set in stone, and then collaboratively fill in details, often sharing them using a wiki or google document. I get very frustrated when players don't engage with this important information.
Now, I'm not holding Storytellers blameless on this. Every ST has fallen into the trap of writing an encyclopedia worth of content for a three session game and expecting players to know it backwards and forewards. I have recently begun to hold myself to a design principle based on the first page of Grant Morrison's All Star Superman, which sums up the essentials of Kal El's story in 8 words and 4 pictures.
But even that sometimes seems to miss the mark. Overall, as a Player, try your best to actually engage the game world content. Don't treat it as pointless or irrelevant. Read the backstory, look at the map, or at the very least, create a character that doesn't know history or geography if you want to not do so. Briefing history experts on basic history content "So George Washington is...?" slows down the flow of game and makes it less fun for everyone.
Meta for Good, Not For Self
Metagaming is a controversial subject and may get its own blog post at some point, but as a player, there are some overall guidelines. Metagaming is not always bad, especially if doing it improves the game for other participants and does not benefit your character too much. This is something best provided by example. I've had multiple occasions where a character wandered off mid action. The rest of the party was engaged in some dramatic scene that wasn't the specialty of the character and they decided that their character would leave and go elsewhere because it wasn't 'in-character' for them to stick around. This made it extremely difficult to maintain the game because that demand of focus split up what would otherwise be a really dramatic moment in game.
In this case, a little meta-gaming would allow everyone to remain part of the game session for a scene and wouldn't create any advantage for the player in question. Another model of metagaming comes in the engagement with the pace of plot. Players can try to achieve too much in a single session in a way that would skip over important parts of a story. Recognizing that progress is sometimes throttled in order to provide opportunities for focus sharing or to preserve a longer narrative thread can make it so more people can engage with things and help others. But sometimes this takes an understanding of the meta-aspects of game.
Now, don't ever meta-game for your own benefit. If anything, meta-game against yourself. If you the player see someone invisible in the room, don't restrict what you say, get really explicit. Spread information around and make it interesting.
CvC, not PvP
Games feature conflict. That's not news. But it's important that the conflict stay within the narrative. Many games fall apart because the conflict within the games becomes a conflict between players. The distinction I've seen in terms of terminology revolves around the term PvP. The term PvP refers to Plaver versus Player and is often used to distinguish games where conflict is commonly between player characters. However, I prefer the term CvC, Character versus Character. Something that is important to remember as a participant in a game is that all the participants should be working towards the same goal, telling a good story. When the characters get into conflict with each other, it should be in the service of story.
What this means as a player is two fold. One, Players should be selective about the conflict between characters. Not avoidant, but try to make it a matter of character tension, not a matter of player tension. Don't agitate against a player you don't like, follow a narrative reason for conflict. My own tendency is to choose my friends as my enemies.
Two, when a conflict comes to a head, make sure that you keep in mind that the person on the other end of the conflict is a person too. When you lose, even if your character dies, recognize that the fellow player didn't set out to hurt you, but that it was part of the narrative conflict. Even more important, when you win, recognize that the fellow player has been a good sport about things, give them that respect. Those actions can keep things from becoming too acrimonious between players.
Trust your Fellow Participants
Which ties into the larger principle of trust. In an ideal world, we'd all trust each other implicitly and give nobody a reason to distrust us, but we're often in a position where we don't really know our fellow participants very well. In these positions, I think it's very important to extend trust to people and try and craft an environment where everyone can work together. Try and find ways to build trust with other participants and treat everyone with respect.
Trust is one of the most essential elements to a game and there's a degree to which the level of trust limits the level of engagement in the game. Part of that is being clear when people violate your trust, but part of it is making sure that you don't violate other people's trust. Ultimately, one can only be accountable for one's own behavior, but as a player, that means that you should follow the golden rule of trust and not do anything that would violate someone else's trust.
Communicate with Your Storyteller!
Which also translates to the Storyteller. While this is one that I think sometimes depends on the player-ST relationship, but I feel that it's important to keep your Storyteller in the loop. As a storyteller, I can't set things up for your plans to succeed if I don't know what they are. I can't facilitate you having the dramatic confrontation with your nefarious uncle if I don't know that you want one. Some STs use foreknowledge of plans to screw over PCs, but that's not me and that's not something I recommend. Instead, being open with your ST allows the ST to set you up to succeed or fail or at least have an interesting experience.
And if there's something you don't like about what's going on, communicate with your Storyteller. I've had many games fall apart over issues that nobody mentioned. Storytellers aren't omniscient, we don't know everything that's going on in everybody's minds/experiences. If there is some aspect of a game that you want more or less of, if there's something going on that you wish wasn't or something not happening that you wish was, let the Storyteller know. If someone is ruining your experience, let the Storyteller know!
And if they don't take you seriously, or treat your statements with contempt and disrespect, leave the game. But a good Storyteller will want to respect the needs and wishes of the community. They might explain to you some reason that things are done a certain way, some aspect of pacing or experience with a given technique that explains why they've made a certain decision, but they might also say, wow, I hadn't thought of it from that angle. That's a very good suggestion, I'm going to think about that and maybe incorporate it. But please, communicate with your Storyteller.