A nice long running campaign spread over months of real time is all well and good, but there are times when what you want is an exciting adventure that can be finished in a single session.
That seems to be all I run these days, since my gaming group gets together irregularly. We can't really depend on being able to continue an adventure later, or expect players to remember what was going on, anyway.
Running one-shot adventures in a role playing game is a bit different from running campaigns. When I first started doing them, the single session would run for seven or eight hours. Over time, I refined the formula down to the point where we can get through a one-shot adventure in three or four hours, and enjoy it just as much.
Here are some of the key points to running a successful one-shot adventure.
Give Every Character A Reason To Care
You don't have time to develop the "why" behind the characters' involvement in the adventure. They need to hit the ground running, and already have an investment in whatever the adventure hook happens to be.
Since we generally create new characters for each one-shot adventure, I usually handle this by simply saying, "Each character must have a personal reason for getting involved". I'll tailor that to whatever the adventure hook actually is, of course. For example, one of the Call of Cthulhu adventures had the hook that the characters were going to spend the night in a haunted house. So each character had to have "a secret reason for spending the night in the house".
The reasons ranged from looking for rare books to sell to pay for college, to trying to have sex with one or more of the other players so the character wouldn't graduate high school a virgin, to trying to track down documents that would incriminate a character's father.
Players are pretty darned creative, so let them make up a lot of the back story for you!
Give The Characters A Chance To Talk, But Not For Long
I like to start my one-shot adventures by giving the characters a chance to talk. Maybe it's just on the drive to the haunted house, or on the boat ride out to the island, or whatever. But that in-character conversation is a great way to get the players into the scenario, as the characters talk about why they're there and start the process of trying to discover other characters' secrets.
This can't go on for long, though. You've only got three or four hours, so give them about five minutes of in character conversation (ten if they're having a lot of fun with it), and then have them arrive where they're going, or have their main contact show up, or whatever it is that actually starts the plot going.
Build Up To A Crisis Quickly
You need a good crisis in the first half hour of play.
The danger of not having this crisis is that the players get into a relaxed mode of playing. They're thinking in terms of taking their time figuring out what's going on, and not in terms of any time pressure. The first crisis generates the sense of time pressure, that they cannot simply take their time to work out the problems, but must hurry!
I run a lot of Call of Cthulhu, so a good crisis is easy enough to generate. A contact is horribly murdered, or a character discovers something terrible, etc. The first crisis in these games is generally the first indication that the characters have that All Is Not As It Seems.
The Mountain Should Come To Them
Many campaign adventures have the characters researching and hunting down clues to what is really going on, and locating whoever is behind it.
In a one-shot adventure, you don't have that kind of time. Throw the characters into the midst of the plot, and have the villain be present, and annoyed at the interruption. Characters may not know exactly what is going on right away, but their adversaries should continue to move the plot along regardless.
Be Willing To Sacrifice Characters
It's a one-shot, so nobody has a great investment in their character's survival. So be willing to kill them!
Don't do it too early, otherwise you have a player who is just sitting around heckling everyone else, but at around the two or three hour mark, be willing to have characters die in order to underscore the urgency of solving whatever problem is going on.
I never kill characters out of hand, but the danger of the adventure does increase as time goes on. Note that a good injury and heroic save works just as well to generate the feel or urgency. So if you don't like killing characters, allow them enough rolls to save themselves.
For example, in one Call of Cthulhu game, a character was tossed out a window by a Deep One. The window happened to be about five hundred feet above a rocky coast. He missed his dexterity check to avoid being thrown out the window, and missed the luck check to catch hold on the way out. I wanted to give the rest of the characters a reason to stick around and give the Deep Ones a chance to whomp on them, so I ruled that the character had caught hold of a jagged piece of glass, and was losing hit points each round.
That kept the other characters in the room, and trying to make it past the Deep Ones to get to the unlucky fellow before he passed out and fell to his death.
One-Shots Are Fun
So if you've never tried a one-shot adventure before, I highly recommend it. You can easily pack a lot of excitement into a four hour session.
When you give it a try, let us know how it went in the forum!