Walter Matthau, who lost family and friends in the Holocaust, only agreed to travel to Germany to shoot the opening Oktoberfest scene in "Hopscotch" (1980) on condition that his son, David Matthau, was cast as Ross. After agreeing to be filmed in Germany, Matthau resisted filming scenes showing him drinking at Oktoberfest until his stepdaughter Lucy Saroyan was given the role of Carla, the pilot who flies him in her seaplane. After arriving at their destination and being paid, she tells Matthau's character that he reminds her of her father. His response is, "That's always been my problem."
The Oktoberfest scene was not staged. The filmmakers hid cameras to prevent people from knowing they were being filmed.
According to the DVD special feature "Introduction by Neame & Garfield", Matthau's agent suggested the film include music by Wolfgang Amadeus Mozart, as this would be pleasing to Matthau. So they made Matthau's character Kendig a lover of Mozart's music. Conductor and arranger Ian Fraser located many pieces of Mozart's music but was at a loss for the scenes where Kendig is typing his book. So they asked Matthau if he could make any suggestions and he obliged by finding some (although director Ronald Neame said that conductor Ian Frasier found many of the Mozart pieces that fit the movie).
Hermann Prey's singing of "Non Più Andrai" highlights the antics of the old biplane as Myerson (Ned Beatty) is shooting at it. The song tells how Cherubino ("little baby"), going into the army, will no longer be a dainty favorite, just as 5-foot-7 Myerson is going to lose his power at the CIA. Also, the song describes bullets flying and even bombs exploding. There is also the aria "Largo al Factotum" from the opera "The Barber of Seville" by Gioachino Rossini. Matthau sings this as he passes a border checkpoint. The words to the aria explain how everyone is looking for the barber, and he moves fast like lightning. Kendig has the aria "Un Bel Dì Vedremo" ("One Beautiful Day") from "Madame Butterfly" by Giacomo Puccini playing loudly on the stereo as the FBI and CIA shoot up Myerson's wife's house. The operatic contrapunto adds a surreal air of ironic justice to the events as Madame Butterfly sings how she will hide from her husband. (IMDb/Wikipedia)