Well, part of what actors do is to develop backstories for their own characters. And in general, they have to be focused on the characters they play as opposed to the characters played by other people. So it stands to reason that an actor would have more insights into his or her own character than into the other characters. That's their job -- to know their own characters inside and out. Knowing all the characters equally is more of a job for the writers, producers, and directors.