As I type this, I find myself in the midst of preparations for a one-shot: a single session game to entertain our group of miscreants for a single evening while our current GM conjures up the next branch of our main campaign. I'll be running a pre-written scenario and I had to ask the question of whether or not the PCs would be built by the players or pre-made by myself. For the most part, the players will get to make their own barring one or two people new to the system I'm using. This leads me to yet another question: If I'm going to make a character for another player, do I need to hand him a background for that character? Or more importantly, if the scenario is pre-written and it's only one session, does the background material really matter?
Admittedly, I have no experience running pre-written modules, but I am aware that tweeking the narrative to better suit your party for things like this is not only perfectly fine, but also somewhat expected. Since this isn't an ongoing campaign, however, what more is needed other than a narrow excuse to get the ball rolling?
Just to make things clear, if you signed up for a one-shot you're probably already prepared for the plot to be basic or at least rather straightforward. The possibility exists that your character's background may have very little to do with what's actually going on unless your GM specifically set out to make it so. Taking that into consideration, how much background do you need for a character? Enough to explain how he/she can do all the amazing things that the character sheet says they can? Enough to give an inkling of personality? A complete life story including that one plot hook about that one time the big bad scary dude killed your parents and you swore revenge on him? Would that be too much? I don't know...
At the end of the day it may not matter. As long as the game goes on and everyone has fun, who cares if you wrote three sentences or thirty pages of fluff? On the other hand, depending on how the character is played, that bit of history may come in handy to the GM. Let's just say for sake of argument that the plot has the party eventually confronting some villain. If the scenario doesn't present a ready made motivation for the PCs to do so, they may not take the hook. That's where the background info may come in handy. Maybe the villain in this story is that dude who offed your family. Maybe in a PCs background they have expressed a certain measure of greed? Then this villain is worth a lot to the local authorities. That or he's likely in the same locale as a boatload of treasure. That little bit of background could be used with some clever improv to lure the PC in the right direction.
Is it necessary? Not in the slightest. Is it handy? Most certainly! At least that's my take on it.
For the sake of a one-shot I'd say it's worth investing at least enough to establish a personality or a motivation. Preferably both.
What of the pre-made character? Do I write up a background for that? There's all kinds of players out there. Some need some context to a character, while others go right off the cuff. I kind of like to leave the finer points to the player to fill in, like the name and gender, even the general description. On a psychological level, they feel more invested in the character if they have some hand in it's creation. Not much investment is needed for a one-shot, though. Just enough to keep people from running the PC down an elevator shaft and mucking up the experience for other people. Even if I left that much for the player to do, should the rest of the background be ready and waiting? The only answer I can come up with is that this is purely situational. I may come up with something simple as a 'just in case' measure, but I'd still let the player have the chance to do something on their own. Better to be prepared, I guess. This'll be another one of those trial and error things.
Let's hope just this goes well, and all this speculation will prove unnecessary anyway.
Be prepared and Stay Classy!