It happens — one of your players who is integral to the plot that night doesn’t show up. Or you have a handful that are unreliable, or there’s a period of time that players get flaky on showing up (for us, it’s usually late summer when one that works in the movie industry is usually on set and another is doing the convention rounds; or it’s the holiday season.) So what do you do?
There’s the usual stuff — don’t play ’til you have the necessary folks for the adventure, have someone run/roll for the missing characters (this is my usual MO), play a board game or pick up game instead. One of the threads over at Gnome Stew had the idea of running a game or series of gmaes con-style — one shots that don’t require a lot of planning or character work.
Think of it as a sort of James Bond style game: You know the hero is going to be a certain way — we don’t have to delve into his weaknesses beyond the most basic — he’s a sucker for dames, his lovers don’t get past the end of the film, he’s relentless to bad guys. The Bond girls — there’s a femme fatale, one or two love interests that make themselves useful in some manner. There’s usually a local sidekick (Felix Leiter, Matthis, Tiger Tanaka, whoever…) The bad guys are fairy simple and have a nefarious plan. Come up with a few action sequences to string the exposition into. Go.
For the GM, think of it as writing a movie, rather than a series or series of books. There doesn’t have to be a lot of backstory, just enough to do the job. (Think Raiders of the Lost Ark — Indy’s really not a well-defined guy: he’s a cruise missile that goes after his prize without hesitation. He’s scared of snakes. He’s got a nemesis that usually bests him [Belloq]. He’s an eminent archeologist with friends all over the world to help him. He likes to use a whip and loves Marion. Done. Go.)
In this style, the GM writes up characters to fit the plot and has a basic notion of what’s going on. The James Bond system had a nice random mission generator in the For Your Eyes Only splatbook, and I’m sure other game genres have something similar. Keep it simple and keep it fast. One idea could be randomly giving characters to the players. I thought that using a system like Leverage or FATE where you could craft up archetypal characters for the genre and just let the players fill in the necessary bits of development on the fly might be conducive to this style of play.
Say, a fantasy game: You’ve got the main hero(ine) and their sidekick (a thief, usually, in the fantasy movies), and maybe the old wizard/sorceress to aid with esoteric knowledge exposition and handling magically stuff. Fill in the blanks — is the lead a bruiser like Red Sonja or Conan, a bit more trickster with animal friends like the Beastmaster, a wet-behind the ears but game for anything type like Perseus or Hercules? (Or Luke Skywalker, for that matter…)
Mix and match tropes, kill some stuff, a chase scene, big showdown with the bad guy that honked your team off for whatever reason…that’s a quick night or two’s play.