Not a movie, actually, but a stage play. The movie adaptation of Gigi never used Audrey, much to the disappointment of the production exec's. Anyway, Gigi (the play) was Audrey's first major role, followed very quickly thereafter by Roman Holiday. For the first time (with the cheque advance given to her) Audrey and her mother had enough money that they could live comfortably since the end of the war. Prior to this they had both been in a small London flat, living off of Audrey's chorus girl and modelling paycheques, which didn't provide stable income. Between her minor film appearance in Monte Carlo Baby (1951) and the beginning of Gigi rehearsal (a bit later in '51) she had quite a few weeks of binging on chocolate, her comfort food. Upon her arrival in New York, director Raymond Rouleau was aghast at her weight gain and demanded to "get the girl (we) hired back."
I don't have my copies on hand, but the information that I know off the top of my head can be found in the biographies which I've provided a citation for below (hope you don't mind MLA format ;D)
Ferrer, Sean Hepburn. Audrey Hepburn: An Elegant Spirit. New York: Atria, 2005. Print.
Spoto, Donald. Enchantment: The Life of Audrey Hepburn. New York: Three Rivers, 2006. Print.