Firstly this role was written for Merilyn Monroe, which were too different character, compare with adorable Audrey Hepburn.
Secondly book doesn't really have a storyline, it is more like incomplete memories about relations in between Holly and Paul.
Third and the most disappointing for me that the book has totally different final. It is not that romantic happy end from the movie with rain, taxi and the cat. In the book Holly just left New York, to look for her happiness somewhere far away in Brazil.
I think script writers has done amazing job to make from medium standard story such a magnificent movie. And definitely without Audrey Hepburn this movie will not be the one, which millions of people love with all their hearts.