Okay, you're a Player Character, a PC (though not necessarily a pc one). Why?
What motivates you to follow the presuppositions of the game? What, for that matter, are the presuppositions of the game? Ask your GM. If you don't get a meaningful answer, apply the Soft Cushions. If that doesn't work, bring in the Comfy Chair.
(If by any chance you are unfamiliar with that particular moment of Pythonism: Torture the bastard until you get an answer that makes sense.)
Start with some basics:
Off-the-cuff example: It is the 1,845th year of the Malachite Throne. Elena Voronek is a fresh-faced 19-year old .. erm ... sorceress. Quite possibly 1st Level with Read Magic and three crappy spells rolled randomly. She was born in 1826.
She comes from the Darvesian Isles, an archipelago of balmy
wooded islands. A mild temperate climate, so she's not used to
long bitter winters. She's happy by the sea, though, probably
accustomed to boat journeys from a tender age, unlikely to be a
non-swimmer (something to think of when selecting skills or
proficiencies). So, all-knowing and wise GM, what else can you
tell me about the Darvesian Isles?
"Erm, well ... they're islands..."
Really? Are you sure? As in utterly really and truly sure?
"Yes. They're an archipelago of balmy wooded islands
with a mild, temperate climate."
Economy based on fishing, I suppose?
"I'd be suprised if it wasn't."
And boat-building, if there's plenty of timber to hand.
"I should imagine so."
Sounds like a rather self-reliant community to me.
"Since it's a bit isolated it doesn't have much
choice."
Relaxed, easy-going, an idyllic little corner of the world
to grow up in...
"Oi! It's my world, not yours. The Darvesian Islanders
are devout followers of the Old Church, their lives governed by
pious observance of the Laws of the Fathers."
Just the kind of place a headstrong young girl might want to get
away from.
"Very possibly, especially if there was some little
extra to push her away from home."
Hmmm, those Old Churchers are a very moral lot, aren't they?
"Are you thinking with your knob again?"
Not necessarily. What's happened there recently? Something
that might drive her out into the world.
"Um, nothing much..."
What's happened there recently? Something that might drive
her out into the world.
"Um, nothing much..."
What's happened there recently... <repeat ad nauseam>
...that might drive her out into the world.
"Well, I guess the Wolfships might have raided."
And when would that have been?
"Hang on, I'll check my notes ... er ... 1813."
So she fled a whole thirteen years before she was born? Well
smart. I know she's got an Intelligence of 18 but...
"The raids started in 1813, prompting a retaliation by
the Throne Imperial. The Wolfships were finally defeated in the
Battle of Cape Endland in 1839, by an Imperial fleet
under..."
So maybe they raided the Darvesians a year or two earlier? Like
maybe 1837 when she was just 11?
"... and his two fellow admirals, with assistance from
the pirates of the Forgotten Coast..."
That's it! The whole community wiped out, except maybe a
handful, and when the Empire's ships turned up, they decided to
take these half-dozen lucky souls back to the capital.
"...which by this time were fitted with arbalests both
fore and aft, and with three banks of oars could..."
Where she was singled out for a career with the Guild of
Sorcerers. Is that possible? How do people get to be sorcerers in
this world?
"... set the waves alight, so the battle is also known
as the Day of the Flaming Waves, commemorated by ..."
OI! How Do People Become Sorcerers In This World?
"Um, they just apply, really."
Oh. They don't get chosen for their apparent gift with the
art or anything?
"Can't say I've really thought about it. Possible, I
suppose..."
Thank-yew!
The Adventurers
Player characters are adventurers. They are people who
have adventures. They get up in the morning, adventure until
lunchtime, have their sandwiches, do some more adventuring, then
stop for tea. There may be more than five in the party, they are
not necessarily famous, and they don't always have a dog.
Assuming that the game world allows for that kind of activity, just why should you want to indulge in it? It's dangerous, uncomfortable, unpredictable, and frequently just plain unpleasant. It takes you to all manner of out of the way places far from civilisation, where a nasty end might hang on every step you take. So, why do you do it?
Possible answers:
There's not much scope for lofty motives in 'adventuring', since it is by nature a pretty selfish occupation. Full-time adventurers are also likely to be some way outside the normal social order, probably walking a fine line between legality and criminality. It is not really an activity for the likes of priests, merchants, court officials, honest artisans etc - they all have jobs to do.
It also implies that you have more or less severed any family ties you might have, so you should ask yourself why? Have you disowned them? Or maybe they've disowned you? Maybe you don't have a family to disown, which might make for a useful bit of background.
The Quest
The PCs have a mission to fulfil. So you should ask
yourself: What is this mission? Why am I in on it? Why have they
let me tag along? What do I hope to see happen if it all works
out? And why have the Wise Ones entrusted the fate of the world
to a bunch of low-level characters?
You have a number of possible roles to fulfil:
Since - in theory - there is a finite end to your questing, you can be a responsible member of society. Either you succeed, and can go back to your steady job and your loved ones, or you screw up and have nothing worth going back to. So the quest is perfect for all those characters who don't really qualify as adventurers. Priests and paladins aren't really cut out for adventuring, but they're excellent quest material.
But once a quest is underway, it is also ideal for adventurers, who can just sort of get roped in at some point. A really long quest might end up being completed by a completely different party to the one that first set out to fulfil it, as the personnel roster slowly changes. And as every GM knows, quests can be really really long, a kind of mythological paper chase after an endless succession of mighty artefacts that can't actually do anything until they're all brought together and buried in a swamp on the far side of the world.
The Rebellion
Pretty straightforward, at least as far as the basics
go. The PCs are all rebels, caught up in the maelstrom of history
in the making. So you ought to make it quite clear: Why are you
rebelling? Who are you rebelling against? (Ask the GM: Whaddaya
got?) What is the long-term strategy? There really ought to be
one, like restoring the Rightful Ruler. Also, what are your
chances? Because if the odds are stacked against you, you really
ought to have more than a little bit of conviction in your cause.
Otherwise you'd have buggered off somewhere safer (and become an
adventurer, perhaps).
Possible motives:
This scenario opens up a lot of possibilities. A really nasty usurper can persecute almost anyone: nobles dispossessed of their lands and titles, priests driven into hiding for speaking out against tyranny, dwarves turfed out of their mountain halls - anyone. Adventurers can get drawn in by the prospect of loot, mercenaries might even be hired.
Another advantage, from the GM's angle anyway, is that rebellions are very hard to win, so the game might drag on indefinitely. Even if the PCs find a clever way to almost win, the balance can tip back the other way and put them back more or less where they started. On the downside, there's not much scope for travel and sightseeing, so the PCs are liable to end up in one corner of the map and the GM chafes at not being able to use all the stuff he's written up for the other bits. So a minor quest might be in order, to acquire something useful for the rebels (maybe picking up a few adventuring PCs along the way). "And when you get back home, you find the rebellion has fared badly, losing much of the gains it had made when you left..."
The Secret Society
There is a clandestine underground organisation, of
which you are a member. What does it exist for? What are its
aims, what is its credo? Presumably, as a member, you believe in
what it is trying to achieve. Why?
A Secret Society campaign can be tied in with the other campaign themes above, especially the Quest and the Rebellion. Or you might belong to the Secret Society of Adventurers, secretly sidling into town in full armour and bristling with weapons with a mule cart dripping treasure. Remember that if anyone asks you if you're a member of the Secret Society, you should answer in the affirmative but ask them not to pass it on because it's a secret.
The very best Secret Society exists for one purpose and one purpose only, namely to beat the crap out of the Rival Secret Society who in turn only exist to beat the crap out of the Secret Society you yourself belong to. No one's going to care what you do so long as it's all in a good cause (beating the living crap out of the Rival Secret Society counts as a good cause).