Actor Allusion: Paul's "decorator" says to him, "Well, love has found Andy Hardy." Andy Hardy was the lead character of a series of films (including Love Finds Andy Hardy) portrayed by Mickey Rooney, who plays Mr. Yunioshi.
Adaptational Sexuality: The gay, unnamed protagonist of Truman Capote's novella becomes the film's straight love interest.
This is Lampshaded in an episode of Seinfeld, where George joins a book club where he is supposed to read the novella but is too lazy too (despite its brevity). He rents the movie instead and is humiliated later on when he mentions that Holly hooks up with Paul at the end.
Bad Dreams: Holly gets into bed with Paul and talks in her sleep about, apparently, losing her beloved brother in the snow. The trope is slightly averted in that she'd already revealed, earlier in that scene, that she worries about her brother, but not the extent to which it distresses her.
The Capital Of Brazil Is Buenos Aires: Almost averted, but not quite. Holly does learn Portuguese in order to go to Brazil, but she nevertheless pronounces her Brazilian fiancÚ's name, JosÚ, as if he were Hispanic. The pronunciation should be Zhoo-zeh, not Ho-say.
Worse, JosÚ even mispronounces his own name in this manner. The actor playing him (JosÚ Luis de Vilallonga) is Spanish.
Hooker with a Heart of Gold: Holly, though this is strongly toned down compared to the novella. She still refers to her "playing the field" though.
Kick the Dog: When Holly abandons her cat in an alley, it's the ultimate sign that she's selling out for a soulless life of luxury. And going back to find him is her redemption.
Little Black Dress: Holly Golightly's dress is probably the most iconic example of the trope in film history. It was designed, like all of Audrey Hepburn's costumes for the film, by Hubert de Givenchy.
May-December Romance: Holly's husband was much, much older than her, and had several kids before marrying her. Holly was still a teenager when they got married, and it sounds like their marriage was relatively innocent, with her doing nothing but sitting around at home all day.
Meaningful Name: What better surname than "Golightly" could there be for a free-spirited ditz with an unserious approach to life?
Most Writers Are Writers: Paul is a writer who has not had anything published in five years since writing a book of vignettes titled Nine Lives.
Smoking Is Glamorous: Ironically, Holly's cigarette stick was supposed to be an affectation that Holly assumes to cover her insecurity. It backfired. Audiences didn't get it, probably at least in part because of the choice of Audrey Hepburn for the role.
Spy Speak: Sally's "weather reports" which Holly passes forward.
Supporting Protagonist: While Holly Golightly is definitely the main character, her love interest Paul Varjak is the protagonist.