Bowdlerise: The movie was originally rated "R" in the United States, but after marketing research revealed that most people who were buying the soundtrack album were young children, the studio re-released it to theaters in a "PG" version with little to no profanity, drug use, and nudity. Wikipedia has a picture◊ of the poster that advertised it.
Defictionalization: The magazine article on which the film was loosely based was bullshit, but Saturday Night Fever ironically became the blueprint for nightlife in many cities.
John Travolta ran two miles a day and danced for three hours daily to get in shape for this film. In the end, he dropped 20 pounds.
Donna Pescow (Annette) gained 30 lbs to better fit the role, as well as training herself back to her Brooklyn accent. (Immediately after the film was done, she set to work losing the extra weight and the accent.)
Enforced Method Acting: When they shot the first bridge scene, director John Badham kept secret from Donna Pescow the fact that when the guys "fell off" the bridge, they actually landed on a platform a few feet below. Badham and the other actors did not tell her about the platform because they wanted a genuine look of horror and anger on Annette's face when Tony, Double J. and Joey appeared to fall off. Therefore, Donna's reaction to them falling, and her facial expressions turning from horror and shock to outright anger, were real, and her next line, "You fuckers!", was not scripted.
Real-Life Relative: John Travolta's sister Ann Travolta appears as the pizza lady, and his mother Helen Travolta appears as the woman for whom he gets the paint.
The Red Stapler: As the main article says, the film created a nationwide craze for Disco music and Disco dancing (together with discotheques), while before Disco was mostly confined to the New York and Philadelphia black and gay communities. Also, Travolta's suit from the movie instantly became fashionable.
When Tony's dad hit him in the back of the head the third time during dinner, his retort of "Just watch the hair!" and then his complaint about being hit on the hair after he had worked on it for so long was John Travolta's own reaction and not scripted, but since it was so in character for Tony Manero to say, it was left in.
The scene where Fran Drescher puts her hand on Travolta's butt was not in the original script. Travolta and director John Badham thought that it would be something that a woman like Connie would do, however, so it was kept in.
Working Title: Tribal Rites of the New Saturday Night, the title of the New York Magazine article that inspired it. The film's title was ultimately shortened to "Saturday Night", as a direct reference to the fact that Tony and his friends inhabited 2001 Odyssey on Saturday nights. However, when The Bee Gees submitted the soundtrack, one of the songs, "Night Fever", was thought to embody the film's spirit better than the original. Director John Badham added the word "Saturday" and it replaced the original title.