It would seem like it would be a good idea to be the good student and do your homework. And to a point, yes. You should have some idea of your characters background. On the other hand, too much detail can be a huge detriment.
Let’s have little thought experiment. Suppose another player says something like “Hey, didn’t I see on your Facebook page that your cousin went to this Uni?” What would be the better thing to do here?
a) Read your notes and say “No, my only cousin died when I was five.”
b) Write down a cousin on your notes and say “Yes, Kaitlin’s writing her doctoral thesis here, but we aren’t exactly on speaking terms. And what the fuck are you doing sifting through my friends on Facebook?”
I would hope most people arrive on “b”. The other option stops a possibly interesting line in the story on its tracks, while “b” opens up options and might lead to interesting roleplay moments. Not only that. It’s a great “show, don’t tell” moment, where your character’s personality is opened up a bit, when everyone hears the character has at least this one awkward situation in their family.
The thing is, as nice as it is to write up your beautiful flower of a character, they are going to meet other players and have to work with them, as well as you are going to have to work with the other players and the GM. They have their own ideas and if you are unwilling to incorporate yours to theirs, because you want to maintain your authorship.
There’s a lot of opportunity costs here. You need to find a balance. Each time you add a new piece of information to your character, it opens certain doors and shuts off others. Therefore, you should make decisions on enough things to get a handle on the character and then add information as you need it.
To get an idea on this, you should look at the first few episodes of various TV-series. These characters receive some exposition so that viewers get some sort of idea about who they are, but not much. We don’t go into detail about their parents, where they were born or how much money they have on hand (unless these things are somehow are integral to the show). Why should you give out any more detail?
Of course, if the GM asks you for something specific, do comply, but they shouldn’t ask for too much either. This is one of those situations where bare minimum is probably all you should do beforehand.