In the end scene when Tony comes to Stephanie's house to apologise, you can see the microphone coming down from the ceiling.
When Tony Manero's father slaps him on the back of the head during an argument at the dinner table, Tony replies, "Would you just watch the hair? You know, I work on my hair a long time and you hit it! He hits my hair!" John Travolta ad-libbed this line. Director John Badham thought it was so perfect for the character of Tony Manero that he decided to leave it in the film.