I think what Areowyn means is more of how the characters personality and reaction to situations and other characters should be. (Correct me if I am wrong Areowyn).
I personally believe that the book should have some relevance, but your character itself will develop with a background and as the story progresses and situations unfold. You have to go more with the truth of the character you want to play than what a handbook or guide tell you. That is the beauty of role playing.... you get to create an entirely new persona. What fun would it be if you stuck strictly by the book.
As Albus Dumbledore once said "You place too much importance... on the so-called purity of blood! You fail to recognize that it matters not what someone is born, but what they grow to be!"
Now... There are, or course, certain rules that must be followed. If you are a Paladin and you hide in the shadows or don't defend the good and just, than, your character should of course change.... But that is at the DMs discretion, as always.