Until recently, I favored huge, expansive character sheets that gave a ton of information and personality traits and everything, but now I'm thinking that that's the quickest way to kill meaningful PC interaction. If everyone knows everything about everyone's characters, what's the point of bonding IC? Even if no one is metagaming, it still ruins the mystery of trying to figure out why a character is the way they are. The best relationship PCs can have is one where they appreciate and want to know more about each other's characters.
I think an ideal setup would be to write two character sheets; one for the main thread, and one to PM to the GM for approval. The first one only gives superficial information--appearance, outward personality, social standing, mannerisms, name, apparent age, ethnicity/race, reputation, whatever someone could discern within three minutes of meeting the character. The PMed CS is the full one, with full backstory, inner personality, motivations, true abilities, etc. Imagine how amazing it'd be if, in addition to the main plot, the PCs were trying to figure out which PC is the subject of the prophecy, or the child of some famous hero, or the vampire.
Obviously this method could be a little bit of a tightrope walk. If a main CS gives away too little information, everyone could unknowingly create very similar characters and cause a game to be unbalanced, and some players don't like the idea of toiling away on a pagelong backstory just to send it to one or two people through the course of the game. But this could also lead to stronger, more organic IC relationships, and more intense drama; if a PC has been dating another character who turns out to be a former terrorist, the PC will react much more realistically, as the shock of the player might bleed into their character's response.
One great CS format I've seen is keeping a character's abilities hidden, but giving a hint through character quotes. For instance, a character who can mind control people might have the quote "I've got everyone wrapped around my finger." Not only does this hint at their powers, but it also establishes their personality, principles, reputation, or how they might solve problems.