For a start the book – a novella really, scarcely topping the 100-page mark – by Truman Capote was set during the Second World War. And the book’s narrator Paul (George Peppard in the film) is more of an observer in the vein of in Nick Carraway in The Great Gatsby and as such is not privy to every action of Holly Golightly (Audrey Hepburn) rather than, say, the redoubtable Dr Watson who, as confidante of Sherlock Holmes, can faithfully record his every action.
So the first task set screenwriter George Axelrod was to update the picture to the contemporary era of the early 1960s. Fashion-wise, this proves a tremendous boon, allowing the director the give Holly her iconic look. And it does permit more leeway with acceptable sexual mores. However, while in both book and film Paul is an aspiring writer, in the book he is initially unpublished, while in the film he has had a book of short stories published, but is living as a gigolo. In the book he is an innocent 19-year-old, mouth clearly agape at Holly’s shenanigans, while in the film he is clearly more mature.
Since Hollywood is intent on providing a happy ending, it was essential for the screenplay to make Paul an acceptable suitor rather than a young swain largely in awe of the captivating Holly.

Axelrod did not have to do much to capture the book’s Holly. In fact, he appropriated wholesale chunks of dialogue. Capote had done such a wonderful job of describing her unique personality that it made a lot of sense to retain her vocabulary and diction.
Axelrod turns Paul into a more dramatic figure, such that he is able to both challenge himself and Holly, emerge from his own self-destructive trap, and develop his own narrative arc, and play a more significant role in Holly’s life, so that the romantic possibilities, which appear distant in the book, can be more easily realized.
The input of other characters is enlarged or diminished. The Japanese neighbor Mr Yunioshi (Mickey Rooney), who only appears at the beginning of the book, is called into more extensive comedic duties by the screenwriter. Mag Wildwood (Dorothy Witney), only seen in passing in the film, has a more significant role in the book, becoming for a time Holly’s flatmate and rival in love. There is no room in the film for Madame Spanella, held responsible in the book for informing the police about Holly’s arrangement with the gangster. For structural reasons, Axelrod is also able to dispense with bar owner Mr Bell, a pivotal character at the book’s opening.
Otherwise, the book acts as a pretty useful treatment, from which the screenwriter need only occasionally depart. Sometimes this is for clarification. In the film Holly insists her marriage to Doc was annulled whereas in the book this is far from clear, leaving her open to charges of bigamy. Axelrod turns into dialogue some of Paul’s observations and turns some dialogue into scenes. In addition, in the book Holly becomes pregnant by her Brazilian lover, thus expecting marriage to automatically follow.
The couple do steal masks from a dime store, but do not visit Tiffany’s together to have the cheap ring inscribed, but the scene has its origins elsewhere in the book. Nor does Paul in the book introduce Holly to the public library and though he finds evidence of her mugging up on South America it is only in the film that that becomes a scene.
The book avoids the happy ending Hollywood was so desperate to reach. Holly goes off on her own. The cat is chucked out of the cab and although a remorseful Holly immediately chases after it, she is too late.
The notion that a creature as wild and individual as Holly Golightly would submit to marriage to an impoverished writer seems a fantasy too far. Unhappy endings were not unknown in Hollywood, look at Casablanca, but for whatever reason Paramount or Blake Edwards dictated otherwise.