Acting debut of Sophie Marceau, who was discovered late in the process by casting director Françoise Menidrey after the then 13 year old with no drama training whatsoever had just days earlier joined a child modeling agency to earn some pocket money and auditioned by chance. The role immediately propelled her to international superstardom and she has since been consistently named as one of, if not the favorite celebrity of the French.
It was during filming that director Claude Pinoteau handed his young lead actress, 13 year old Sophie Maupu, a map of Paris, suggesting that she use it to choose a stage name, so the girl chose the Marceau avenue, to have the same initials as her real name, and thus became Sophie Marceau.
The film was not expected to have much success at release, debuting at the sixth place of the French box-office, but admissions kept growing day after day, eventually reaching and staying at the first place for a month and remaining in French theaters for 35 weeks, or over 8 months, becoming the most successful film of 1980 in France with 4,378,430 tickets sold, over Star Wars: Episode V - The Empire Strikes Back (1980). This was just that start of a major international success, with over 15 million tickets sold across Europe. By the time the film reached Italian screens in late 1981, the soundtrack's single "Reality", had been a huge hit for months and the film became a phenomenon, becoming the biggest film of the year in Italy, beating out E.T. the Extra-Terrestrial (1982) with nearly 8 million admissions, almost double that of France.
Claude Brasseur and Sophie Marceau play father and daughter here and in the sequel, The Party 2 (1982), but would reunite six years later to play lovers in Descent Into Hell (1986).
Screenwriter Danièle Thompson got the idea for the film by coming home one afternoon to find her then teenage daughter Caroline Thompson having organized a party at their home without telling her. She initially conceived the project as a potential Happy Days (1974)-style TV series.