So, I've been thinking about this one for half a day. And my gut feeling is still kind of about my initial reaction of
"Nothing so big can have a single key".
Although I guess everyone's got their own particular angle that'll let them in into that big problem, and this might be the way for some.
That said - while I get the angle of looking at "What do they want" and believe its big and believe I should ask myself that more often...
... how am I meant to know exactly how far they're willing to go and what they're willing to lose? Or maybe the advice is to show those things to the readers, not to know that at the outset.
Exactly this. It's like I said; you don't imagine your characters so much as you channel them. Real life experience, it is said, is a vital ingredient in improving the quality of one's writing, and that is because on the basest level, it is an outlet for our emotions, repressed and expressed, dark and light, etc., and so, those are the qualities that are built into each character you build in such varying degrees as to make them come alive. Then you can simply see where they take you; after all, how many times have each of you, in the middle of a piece, start to realize that the story may not be going the way you initially planned, and that the involved characters may not be behaving the way you intended?
One example I have of this is of identical twin sisters of mine, Rena and Lana Maeshalanadae. (Don't ask about the family name; it's meant to be a jibe at classic D&D style fantasy, in the vein of Forgotten Realms and such.) Initially, Lana was the one I wanted to be a little more forward, rebellious, etc., the one more intended to be putting herself out there, while Rena was meant to be the one to be shy, withdrawn, classic bookworm type, but ultimately I discovered right off that I had switched their roles, so I simply went with it and I think it's worked out. Rena is a technomancer who is the classic cybernerd, but she's also popular, outgoing, and athletic, participating in various physical activities, namely running. Lana is the shy loner who doesn't have many friends and tends to get manipulated by her sister into going into things.
Ultimately, there's various ways things can go, simply by changing names and pronouns at key points. And who knows? Maybe it'll be made more interesting if you make those tiny little subconscious mistakes. The characters are those emotions taking over in a medium where you have absolute control and where nobody can be physically harmed by any form of violence or mature content within. (Psychological perhaps withstanding, considering age or mental state.)
Just let things go, and it's up to you in the second and third and fortieth drafts what you really want to happen, and what you might take a second look and go, "Hey, that's very interesting..." as you proofread, even if it isn't what the initial idea truly was.