Flynn is definitely trying to NOT be an omniscient author- the kind of author that "knows" and henceforth writes in every angle(thoughts, conversations, actions) of every character in the book. For an average reader who only turns the pages to kill time, that kind of style gives them immediate satisfaction of obtaining information whether you need it or not. You don't have to think at all, you don't have to guess, you are just simply absorbing the opinions, the perspectives of the author or those portrayed through the author.
We all need that kind of pastime from time to time. But you can't sit down and just read with Flynn. True be told, she played fantastically the "I can only describe what I see, which is what you see, readers" card. She keeps you on the edge of your seat all the time. Seemingly I was being told everything going on in the head of a clever and pretty woman who was just so cool. The diary , the presented diary of Amy, generates the effect exactly the character has intended, which is altogether the effect the author has intended. Of course, the whole thing is manipulation and the reader is so thoroughly manipulated in their perception. What we think of Amy, the protagnist, is what she wants us to think of her, which effect is also the author wants to achieve by writing Amy and writing Amy's diary ,essentially. However clever Amy is in her schemes to deceive the police, the neighbor and ultimately the reader who observes her with the angle she allows you(or the author allows you?), the author- Flynn can only be more clever, right? to design a character who design a scheme as seemless as that.
The writing philosophy aside. Flynn gives us a way into the world of created persoanlities, of a highly-intelligent woman who invests her energy to study what makes popularity and what creats favorable opinions. She can change her personalities, or the perceived personalities based on her study of what works and what does not. Yes, the only thing she can pull off is genuineness and true fearless of facing the world with her whole true self, but she doesn't care. Such persons are among us and there is an Amy, perhaps, in every smart girl who knows how to read peoples mind. She becomes the way she is, of course partially due to that she always had a double life- a life for real and a life in the book series her writer parents created. She learned too early how to control perceptions.
Flynn is vividly insightful of the gender difference as well. The two completely different points of view, or mentalities of a man and a woman in marriage. She has to know men so well. She has to know women so well. Her deep sympathy out of knowledge of the intrinsic emotional needs of the two opposites in a marraige, can change your view of marriage.