As the title says how much information do you know about your PC(s) when you start playing?
I'm mostly speaking to things that may not be captured during character creation.
Some game systems aren't concerned with pre-establishing:
I think I have found that I cannot play a character until the above details are sketched, at least roughly. If I don't have a strong feeling about any of them I use oracles and random tables to fill in the blanks
What's the bare minimum amount of detail you need for your characters?
Are there things that are only established during play?
What tools do you use to flesh out details?
I found in the past that the Rory Story Cubes were great to create short & sweet backstories for PCs or NPCs. Oracles are very versatile, of course, so occasionally I use them for htis purpose. Random tables focused on this kind of stuff are great too (Lifepaths are awesome).
One tool I haven't used, but plan on using at some point are the Backstory Cards (link pending). They're meant for connections between PCs, but there's no reason why I can't substitute another PC for some NPC in my character's history.