In the beginning of the movie, Nick seems to be talking to a psychiatrist. In his file it says that he is an alcoholic, he says that they were all alcoholic at that time. This is very different to the book, in which he describes only having been drunk a few times, “ I have been drunk just twice in my life, and the second time was that afternoon”. I think the producer included in this part about Nick to show how much his experiences in New York affected him. Gatsby's funeral is pretty similar in the book and movie. In both it showed that the people who came to his parties were only using him, and that in the end they didn't care about him at all. For example Nick says ”I found myself at Gatsby's side alone...my responsibility because no one else was interested”.
The one difference though was that in the book Nick manages to find Gatsby's dad,and he comes to the funeral. “It was gatsby's father… the telegram only said that he was leaving immediately and to postpone the funeral until he came”. I think that the producer didn't include this part in the movie because he wanted to emphasize just how alone gatsby really