Martin Scorsese has said that he thought Robert De Niro's best performance under his direction was in this film.
Much of the scene where Rupert (Robert De Niro) shows up at Jerry's house was improvised. Kim Chan improvised his lines when Jonno calls Jerry. The part where Jonno has trouble opening the front door was not planned. Chan really could not open the door, and Jerry Lewis improvised his reaction.
When Jerry Langford is walking down the street, he is stopped by a woman talking on the telephone. When Jerry refuses to talk to someone on the phone, the lady says, "I hope you get cancer". This incident actually happened to Jerry Lewis. According to Scorsese, Lewis directed this segment himself.
Robert De Niro used anti-Semitic remarks to anger Jerry Lewis while filming the scene where Rupert Pupkin crashes Jerry Langford's country home. Lewis, who had never worked with method actors, was shocked and appalled, but delivered an extremely credible performance.
In the scene where Robert De Niro and Sandra Bernhard argue in the street, three of the "street scum" that mock Bernhard are Mick Jones, Joe Strummer, and Paul Simonon, members of the British punk rock band The Clash.
Cathy Scorsese: The daughter of Martin Scorsese as Dolores, a fictional fan of Rupert who appears in one of his daydreams.