Givenchy and Hepburn were pals, but they also helped each other's careers immensely. Givenchy created a bunch of dresses for Hepburn (including the iconic Sabrina dresses) and Hepburn made them look good. (Source)

Audrey Hepburn and William Holden (who played David) fell in love and had a passionate affair. Hepburn wanted to marry him, but they eventually broke up because (a) Holden had a vasectomy, and could not have children, and (b) Holden was already married and his wife wouldn't divorce him. (Source)

Wilder was writing and rewriting the script as he went along. One day he asked Hepburn to pretend to be sick so that he could finish some rewrites. Hepburn did, and fooled the studio, because she's an actor. If you're going to ask someone to feign an illness, get a professional. (Source)