What works on a novel not always works on film. Especially in those times where having a pessimistic ending would have doomed the movie.

In the movie there is a happy romantic ending
, but in the book it does not end happy. In fact it seems sort of melancholy.
That's not really unusual - apparently audiences prefer it that way.

And where was Joe Bell? he had a major role in the book.
He was in love with her.
I feel like they made her fall in love with Paul because that is what audiences expected of Audrey Hepburn who today would be a Jennifer Aniston type. But the real actress to play the part now would be that actress in Hunger Games.
Honestly she can do anything.
It was a novella about a certain type of woman. And Audrey was not that. As an actress she could not have pulled it off.
But they wanted her so they changed the screenplay to suit her. It was a bad movie.

In the book she goes to Brazil.

the whole movie is different because
they were not supposed to have a romantic relationship, he was gay and the kind of interest he had in her is the fact that shes interesting and mysterious and totally reinvented herself upon arrival to new york
I solemnly swear I'm up to no good

That's her character progression/development.
Communism was just a red herring!

I haven't read the book, and I was a little surprised by the ending of the movie. I expected Golightly to just go to Brazil, so I'm not sure how I feel that she didn't. Hmm. Suddenly staying in New York like she did, didn't seem to fit the character. I'm disappointed somehow, here.
It's about how audaciously you are carrying on in calm.

Well, they changed the film radically from the book from what I recall.