I'm really a single-player gamer, I have to admit. Left 4 Dead is one of my favorite multiplayer games because it plays a heck of a lot like a single-player game. To me, playing alone is something of a "purer" experience, allowing me to see the game the way I wish to see it, rather than conforming to another's notion of what a game "should" be like.
That's why it can be so jarring to play a single-player campaign with someone else. I'm not talking about a co-op mode, an MMORPG or the "campaigns" designed as practice matches for multiplayer. I'm talking about playing "Story Mode", "New Game", "The Journey", the stuff that was "meant" to be seen and played by me alone, only this time, with others. That additional presence can make for an entirely different game.
Even at its very simplest, having someone else in the room can change the way you enjoy a game. Playing Ar Tonelico in a public place might have you turning down the volume somewhat while you keep a young girl alive. Playing Saints Row 2 the way Anthony Burch played it might leave you with a little explaining to do if your 9-year-old nephew were hanging around.
Playing with an actual audience, people sitting and watching you play, can infuse an even more pervasive sense of self-consciousness. You start wondering whether the people are enjoying (or hating) Metal Gear Solid 4's cutscenes as much as you are. You feel the pressure as your constant failures to jump a gap in Mirror's Edge may well be boring the audience.
Were they as moved by the death of that Colossus as you were? Did they also notice that you've just passed a critical stage of The Fool's Journey? You want to ask them, enlighten them, help them enjoy or hate the game as much as you are, but are at the same time afraid of what they might think of you, especially if they're non-gamers. Yes, that stigma still exists.
The game can once again be changed completely when your audience comments. A wild shot that fails to dismember a Necromorph transforms "OH SHI- I'm boned," into "Dude, your aim sucks." Edge-of-your-seat fright can become roll-on-the-floor hilarity when a game is seen through more than one set of eyes.
If heckling can change a game that much, imagine what gang-playing a game can do. I've never been much for Silent Hill, in spite of the huge praise heaped upon (most of) the series. Is it because of my cowardice or general disinterest in "survival horror"? Perhaps. But I suspect it's because I played the first game with friends. We ran Silent Hill in a single summer day, always with one of us holding the controller, another holding a walkthrough (in case we got stuck). Playing Silent Hill this way had us attacking the darkness rather than casting our flashlight's beam into the fog.
Play that way, and we are all are the Master of Unlocking (I know that reference is misplaced), we are all an Italian plumber chasing down a cockteasing bitch, and most of all, we're playing solo, but never alone.
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