Here are a few do's and don'ts for convention games. Convention games differ from your weekly table-top session, because you'll be gaming with strangers, you have to develop character, plot, conflict, and resolution in a few short hours - and you may have to explain and enforce the mechanics of the system to people who are either entirely unaware of them, or deeply rooted in their own understanding of them. Both must be accomodated.
DO: Pregenerate characters. Give those characters sufficient backgrounds for the players to get the idea, but leave them vague enough for the player to customize and not be pigeon-holed. Woe unto the person who picks the GM's regular table-top character to play! This happened in a Marvel Heroes game where, mysteriously one of the super heroes was also the GM's D&D character (a, kid you not - ninja/ranger-paladin.) The poor girl who selected this character was subjected to a good solid hour of explanation of her background, her motivation, and many, many of her exploits - all of which were irrelevant to the game we were about to play. Contrast this with say, the game of Buffy The Vampire Slayer crapdaddy and I played in - character descriptions were "You are the Jock." The rest was up to us, with some points left over for customization. (I took "your mother" as a 1 pt. contact, and actually became relevant) Despite the GM's fetishistic fascination with extremely petite women (nearly all the characters were female, and none of them were over 5'0" tall, except the Slayer) this worked to get people into the game quickly.
Don't: Insist on rolls for everything. In the afore-mentioned Marvel Heroes game, we had to make rolls to open doors, to walk across rooms, to search dead bodies for clues. In the Serenity game I played in, we were constantly making perception rolls - which, despite spectacular successes on my part (I was playing Wash) - never amounted to anything, we never succeeded. Nothing kills a game like bogging it down in stupid failures - if opening *that* door isn't a major obstable, then don't allow for the possiblity that the players will fail to do so. Saying, "Because of your incredible strength, you batter down the door - a huge boom echoes through the facility as you stand in front of the twisted wreckage." is quite enough - making me roll a half-dozen times until I get the right number is pointless. Unless it's a time-sensitive thing - can I succeed within X amount of time? The randomizing element of dice should only serve to elevate tension in the game, not to substitute for narration.
Do: Have a plot. Make it clear, fairly simple, and with something on the line that the characters care about. You probably have about four hours, so a byzantine lair of betrayal and deception is perhaps out of your grasp, unless you know who you're running for. (But it's a convention, so you don't.)
Don't: Do absolutely nothing to further the plot other than ask the players "What are you doing?" The answer you'll get will probably not further the plot, since no one but you knows it. Tell the players what the conflict is - then let them come up with how to resolve it.
Do: Give your players latitude in how they'll solve a problem. If they come up with something creative and weird that you didn't anticipate - congratulations, that's the fun of a con game. Roll with it, don't squash them.
Don't: Let "Look at me" players ruin everyone else's good fun. There's always one in every game - and you can't let them steal the spotlight constantly. Everyone should get their turn. For alpha players that like to take the driver's seat, make them involve the other characters. For players that prefer to sit and watch - be sure that key elements of your plot will revolve around their input. In the InSpectres game I played in, there was one girl who was constantly using the free-form narrative elements of the game mechanics to make the whole story be about her character, and her character alone. She tried to put words in the mouths of characters other than her own, she tried to dictate who would put what on their character sheet during set-up. The GM was a modest and polite fellow who did a fairly good job of riding herd on her - but it was still vastly annoying to the rest of us.
DO: Tell rules-lawyers to shut up. You're running the game. You've got a time-constraint, and a story to tell. If you break or fudge the rules, so be it. If you don't use the same house-rules as the mouth-breather that's been playing this game every week for the past 15 years - so be it! Be firm, fair and polite. The rules are a guideline, not holy writ.
Don't: Run a game you don't know well. The "look at me's" and the rules lawyers will run all over you. Be unaplogetic, though, if you don't remember a rule and just make a snap decision to move on. The fun is in the story, not in the mechanics.
Do: Use cool props. Iin the Serenity game, the GM had an amazing sand-table set up, with a stereo built into the bottom - and a fully 3-d, to-scale map and model of Serenity herself to use as the playing surface. He had spare pencils, dice-sets, poker chips - even little costume pieces to hand out. (I used those dinosaurs like no one's business!) It was really neat. Unfortunately, that was the extent of his preparation.
Don't: Give special attention and advantages to female players. With a few notable exceptions, they probably don't want it - and it won't get you laid, not ever. Never. It will, however, irritate those of us blessed with external genitalia, and also make you look very, deeply pathetic.
Do: Feel free to run the same story a coupla three times throughout the convention.
Don't: Tell the players of this session how much more successful the players of the last session were.
Do: Let the players do what they want. Don't skip their initiative in a combat because you decided what they're going to do, made their roll, and moved on without bothering to inform the player. That's wildly irritating. Don't tell me what my character just said or did, either - let me decide, that's what I'm here for.
Don't: Not tell me what's going on, and then punish me for being ignorant of key game/setting/plot points that I have no way of knowing. Telling me stuff my character would know, but that I don't, isn't spoon-feeding me, it's your goddamn job.
Do: Shower. Jeebus, the funk, the funk!
Don't: Let too large a group play. If you can't handle the number of people who turned out, turn some away. If you're already pre-gen'ing characters anyway, you've carefully balanced the threats vs. the characters numbers and competency, haven't you? Don't throw that off at the last second, because someone showed up with a buddy and wants to wheedle a place at the table. Better too few than too many.
Do: Narrate over stuff that's colorful, but non-essential. This sort of goes with not making too many rolls, but it's also about pacing. If the players aren't directly interacting with it, and it doesn't directly advance the plot - just explain it. Sure, make your explanation funny or engaging, but make it quick.
Don't: Let NPC's do all the heavy lifting or overshadow the players.. The players are there to do the work and resolve the conflict. Your NPC's are there to move them along, give them info, or maybe help them out - but if your players are "B" Grade super-heroes on the level of Squirrel-Girl and the rest of the Great Lakes Avengers - don't have them following Captain America around and watch him do all the fighting. When an NPC has to make some aggressive action against another NPC, don't even bother rolling the dice. Do whatever it takes to get them out of the way, and put the players back in the spotlight. This game is about them, not about your super-awesome Mary Sue NPC that you pencil-whipped up to be so freakin' cool. The players get to be the cool ones, the NPC's are there to accentuate their particular flavor of awesome.
Do: Leave time to get honest feedback. Not just "hey, thanks for the game." ask your players what they thought you should sustain, and what needs improvement.
Don't: Make your entire plot resolution depend on a skill check or power that no one has, or that is exceedingly difficult. If the players have made the right moves to get into the right place, with a feasible solution to your conflict - let it happen. A friend railed about a Call of Cthulhu game he played in, in which the whole story stalled because only one character had "Cthulhu Mythos" as a skill, he had it only 1%, and there was absolutely no way to solve the mystery without successful use of that skill. This is mind-numbingly dumb - you've built a literally 99% chance of failure into your story.
Maybe more later - but that's all for now.