It takes a lot more prep for the aspiring and enterprising GM to turn a roleplaying game into a one-shot event that anyone can sit down and play. But come on, why should the tabletop gamers have all the fun?
Here's the proposition: If there's an RPG that you want to run something for, and you don't have a regular group to write for (or you have a regular group that isn't willing to play along), write for them anyway. Create an adventure, and fill it with thrills, chills, spills, cheesy jokes, electrical shocks, and psychic surprises.
Either make it adaptable by accounting for all character types, or create characters especially for the adventure and tailor it around them. (Personally I recommend the latter; see below.)
Then sit on it. Stick everything in an envelope and stuff it in a drawer or a box or a time capsule or something, until such a time as you're ready to unleash its brilliance.
What, preparation? Are you daft, man?!?
Sounds weird, doesn't it? Someone sitting down to plot and write an adventure in a roleplaying game, without knowing who or even how many will be playing? And then pulling it out later at some later date for whomever happens to be around? It's not, really.
The Other Side of the Fence
The board, card, and tabletop gamers have the advantage of spontaneity (gobs of it) and preparation (next to none needed). If you're not doing anything else and the whim hits you, all you have to do is grab the box, lay out the pieces, claim tokens, and do whatever else you'd have to in order to get started.
In contrast, the tabletop RPG contains the rules by which you'd run the game and little else. From those bare, bleached bones, you have to cook up a story, all the encounters and challenges that the player characters would run into, all the encounters and challenges that the player characters could run into if they do things right or wrong, and a whiz-bang satisfying (or admittedly regrettable) ending. To run a game like that spontaneously, you'd need all those details beforehand, prepped and ready to go.
But, again, it's not like it hasn't been done before.
Proof of Concept
Advanced Dungeons & Dragons (yes, I mean that first edition) got a certain amount of extra gravitas from the strength of its published adventure library. 3rd and 4th editions also got some prepackaged adventure loving.
And that's not the only game line that's gotten that kind of support. Yes, the setting lends itself for some perverse action, but would Paranoia have caught on the way it did, if not for the delightfully unhinged imagination and audacious plotting of the adventures published for it? Dig around enough, and you'll find scenarios written for other games too—Traveller comes to mind, and so do many others I've mentioned here entirely too many times.
The Lost Art
Their strength was also their weakness, though: the ease with which GMs could pick up an adventure, skim through it, and put it into action was also the ease with which a player could pick it up, skim through it, and know what details the GM was going to throw at them and when, unless the GM mixed things up beforehand.
Consider also that those published adventures could make no assumptions whatsoever about what characters would be trotted through them, and the feat seems even more daunting. Nonetheless, they did it, by including either contingency information or pre-generated characters.
Gratuitous Allusion to the Start of the Article
They are preloaded for play. All you have to do is unbox them and you can start playing a game you like with other people. And in this case, if you took my proposition above seriously, I'm talking about RPGs here too. And yes, my proposition involves doing your own boxing.
Instant Mayhem—Just Add Players
It really isn't that hard to create one of these things. You're going to do many of the same things you would if you were writing for a group of players that you know very well. There are some things, though, that you'll be doing more of.
But first, there's a decision to make.
Pre-generated characters, or any characters?
The question of "who's playing?" is a major one, and this isn't just about the characters, either. Will the players know the game setting well enough to have their own characters made up beforehand?
In most cases, store-bought packaged adventures generally get around this problem by being very clearly associated with a specific system.
Living Forgotten Realms is a campaign series run indirectly by Wizards of the Coast and more directly by volunteers in game stores and book stores all over. They get around this problem because they make it abundantly clear which system they're playing in.
Paranoia is the exception; each one of those published scenarios and books had pre-gens. And it's just as well; characters are so disposable in that game anyway that it's a wonder they're not printed on flash paper. (Note: Preliminary research indicates that this can be done, as long as you don't try to laser-print. That drum gets hot, y'see.)
If anybody, then everything
Being able to write an adventure for any and all comers is a hopeful, happy, and wonderful thing; it assumes that you're going to be around players of that game. It means that you'll probably have a wide range of characters to play with.
With that possibility comes an additional responsibility: if you're going to have any possible character in your game, then you need to be prepared for any possible character in your game. The party that takes on your challenge (and it should not be adversarial) might act like a well-oiled machine with military precision, or it might be a cross between bumbling idiots and squabbling children. Your plotting has to adapt to either of these extremes.
Oh heck, I'd recommend everything anyway
Now we consider the alternate case, where you have pre-generated characters and know what suite of abilities you're scripting for well in advance. I'd still recommend being deucedly thorough with your plotting and scripting.
First, consider one of the biggest variables in any game: the players. Different players will come up with different strategies even if they're playing the same characters, and they might not find exactly the solution you think would work best. Your plotting has to respond to different methods; note that this doesn't mean allowing them, but it does mean dealing with them appropriately.
There's another variable that you might not think to account for: time. Oh sure, you have a whiz-bang plot with thrills and spills aplenty and beats of pure comedy gold worked throughout, but will you remember all of its particulars and nuances later? Don't trust your memory. Write down all the details, and don't be afraid to write them down a few different places to ensure you can find them later.
Table of contents? Footnotes? Sidebars? Index? Cross-references? Sounds like overkill, doesn't it? But until you know the frustration of searching frantically through your notes for a detail you yourself wrote in there while your players watch you, you won't know how useful these boogers are.
Oh, and because I haven't explicitly said it yet: if you're going to include a pack of pre-generated characters in the one-shot pack, don't forget to include the pack of pre-generated character sheets in the one-shot pack.
Is this spades or hearts? I'm too light-headed to tell.
Now here's a detail you might mistakenly omit: A quick (one-page) introduction to the system you're playing in. You really can't be sure who you'll be playing with, and if it's a niche or rare system, you can't even be sure they'll know how to play. They might pick it up quickly, but rather than risk explaining the same details four or five times, write up the intro which people can read at their leisure.
And once it's done, get ready to do it again!
This project involves a good bit of writing. It doesn't make sense to use it just once, does it?
Save your project files so you can print up another one of these things at a moment's notice. You might not have to print all of it, if the bulk of the printed scenario survives its play-through intact. After you run a set with your self-packaged adventure, print up the parts you need again, and seal it all up in another envelope for the next time you encounter a group of players who haven't had this particular thrill yet.
Once the i's are dotted and the t's are crossed, the ink is dry and the envelope is sealed, you've got a one-shot scenario suitable for busting out at a moment's notice.
And there are more such moments than you might think.
The Substitute Pick-Up Game
The existing campaign might get put on hold for a week, because the GM is sick at home with The Yaws. That's when you might want to open up a can of Something Else To Do.
The Introductory Pick-Up Game
If you know a group of people who know nothing about a particular system, the canned adventure may be a great way to introduce them to that system, to you as GM, and to each other.
The Convention Pick-Up Game
Science-fiction conventions and gaming conventions tend to have tables set up to provide space to play games. The games most enjoyed are the ones with little preparation... not coincidentally at all a lot like the kind you've packed up if you've been following the steps above.
There's a possible side-effect to producing a great one-shot for your favorite game: people who play the one-shot might want to play it again. People who have never heard of said game before may suddenly find they have a new favorite game too.