I had been thinking about this topic for a while, and then yesterday I had a co-worker tell me the story of how the edgy/loner wizard in his Dungeon World game was essentially ruining the game for him (as another player), I listened to episode 4 of Table Top Babble (while not totally on the topic, bumped up against it a few times), and I watched Taking20's latest video.
I'll preface this all with the disclaimer that if you are running an evil campaign, or have a specific story hook (like the party is all from the same family, or are the survivors from the same village, or whatever) then most of this may not apply any you can go merrily on your way. But if you are running a more usual party-of-adventurers style campaign, then this stuff can be pretty important.
If a campaign is going to go on for any length of time, having the characters debate "why should we go there?" or "why should we do that?" with every adventure path will get really tiresome and risk grinding the game to a halt. The characters need to have a built-in reason to do stuff. Most newer RPGs include some of this baked into character creation, but I don't think most go quite far enough. D&D 5e has personality traits and bonds and ideals as part of the backgrounds, but I find that they are not always enough.
Characters need to have their own motivation to be part of the party and to seek adventure. That's it. But how do we make sure that happens? And, since I am (and most players are) pretty lazy, how do we do this as easily as possible?
You could just tell the players that, and leave it there. But let's see if we can come up with some tools to help them get to that goal in a way that is going to stick with their character through the game.
While you can require players to come up with pages upon pages of backstory, not all players find that enjoyable, and it does little to guarantee that the character will have a reason to explore the world and to find how the story of their character is the part of the story of the other characters.
Currently, this is what I'm thinking is a "minimum requirement" for a character questionnaire -- we want this to be as small as possible, but still force the players to think about their character in a way that will make the game better for everyone. Also included are some questions that go less towards character motivation and more towards mine-able material for the GM. If your campaign is less sandbox and more modules, you may not need to have extra story seeds, so feel free to ignore.
1. What key event(s) shaped/changed/influenced/defined you?
2a. Why do you seek adventure?
2b. What people/places/things/ideas are important to you?
2c. How would someone (who knows well) describe you?
3. What do you want to accomplish?
That's it, five questions. Some of them are going to have the same (or related) answers, and that's fine. As a GM, when looking over the answers (or when explaining to the players why answering them is important), here is what I would focus on:
What key event(s) shaped/changed/influenced/defined you?
This is the "cornerstone memory", to borrow from Westworld. Some players are going to have some key backstory events in mind before they start to flesh out their character, but not all will. You can easily come back and think of some key events if you would rather start with things like personality or more mechanical things like race and class. For now we just want the player think about the event(s), and we'll work on making the events mesh with the personality traits and such as we go on.
A character could have several key events, but I think they should have at least one. It might be the time they first saw a magic spell, or when orcs raided their village, or they stood up to a bully, or they stole something to survive. The player doesn't need to decide every last detail of each event, but they should have enough specifics that it makes sense as something that clearly shows how and why it has shaped their current personality and and goals. It should be a turning-point for a character, something that they can point back to and say "that is the reason I am x", for whatever x most clearly defines them.
If the event(s) don't tie into the personality and goals, the GM and the player need to work together to come up with something that does tie them together.
Why do you seek adventure?
This is the one that makes sure the characters have motivation to go out and do stuff. I don't see any reason to beat around the bush with this one, I think it's best to just ask it straight out. We not going to ask if they seek adventure, but why. If the character isn't going to have a reason to delve into dungeons or fight monsters or save the town, then you are setting the group up to have a dysfunctional party. If you've got a non-traditional campaign in mind, you may not need this, but for the stereotypical band of adventurers, this is the most important question to answer.
This is not to say that parties should be a hive-mind of homogeneous do-gooders, in fact different values and approaches are what is going to lead to interesting role-play, but having a reason to adventure is a pretty essential part of making an adventurer. Reluctant adventurers are fine, as long as the reason for them overcoming their reluctance isn't something that is going to be resolved too early on.
If a player can't answer this, they either need to work with the GM to find something that would motivate their character (and perhaps the GM can incorporate such motivation into some of the campaign's initial events), or go back to the drawing-board with their character concept.
This is where you root out the lone wolf, spotlight hog, moody edge-lord, evil necromancer/warlock/rogue, classic chaotic-neutral asshole, PvPer, and such. If you and the players want that type of game, that's fine, but set that expectation up front and make sure everyone is on-board.
What people/places/things/ideas are important to you?
This questions has three goals. The first is to reinforce the previous questions, as the things listed here should probably tie into the key events or the reason to seek adventure.
The second purpose is to give a tie-in to the party, be it its member or its goals. This is where the bonds and relationships that cement the party together need to be defined. Again, this isn't a suggestion that every character is a life-long best friend of every other character, as there's a lot more fun role-playing space with characters that aren't always seeing eye-to-eye or know everything about each other. There should be enough of a web connecting characters that there's some overlap and that a single character isn't the linchpin of the group's cohesion.
The third goal is optional, and that's to give the GM seeds for NPCs and events and locations for the campaign that will have a built-in reason for the the character to care about them. It can be part of the world-building, and give you a lot of material to work with, from individual encounters to story arcs that span the entire campaign.
And again, you want to make sure that there aren't too few things here that if some of them are resolved there's no motivation left.
How would someone (who knows well) describe you?
This is meant to get the player thinking about the character's personality. Again, it should tie into the previous questions. If the personality traits aren't evident from those answers, the player should go back and figure out what they need to add to make it so. There should be a reason a character is studious, foolhardy, brash, abrasive, curious, sarcastic, or whatever -- and if it is their defining trait, what is the event or story behind it?
What do you want to accomplish?
This is the final check to make sure everything is roughly aligned. Not all party members need to (or should) have the same goal(s), but as a GM this is going to give you a good idea of the theme or possible arcs of your campaign. For the players, they can consider if their character's only goal is adventure, or if they have other goals that influence or may be at odds with that purpose. Again, goals here should have a reason for being here, if they don't go back and come up with an event or important people/places/things/ideas that gives them a reason, or change the goal to align with an event or important people/places/things/ideas.
That's it. Five questions, several of which you probably have at least partial answers just from doing the regular character generation. As a GM, they'd give me the framework to make sure the characters are all member of an adventuring group.