In the scene where a large number of nuns are observed queuing up to see Love Story (1970) at a local cinema, the cinema next door is seen to be showing a French film, Rum Runners (1971), starring Brigitte Bardot - and also Clive Revill, who appears as the hotel manager in this film.
Billy Wilder cast several non-professionals in supporting roles. Guidarino Guidi was a Rome actor's agent who called Wilder about getting jobs on the film for various clients and was instead cast as the maitre d' of the Excelsior Hotel; Raffaele Mottola was hired as the film's dialogue coach and then persuaded to play the passport official at the start of the film; and the elderly Armando Giovagnoli, playing the nonagenarian Baron, was Wilder's chauffeur on the film. The famous make-up man Harry Ray, who worked on a number of Wilder films (and was, for a time, Jack Lemmon's personal make-up man), was cast as Dr. Fleischman, the dentist on Wendell's plane - perhaps because of his close resemblance to Billy Wilder.
After Marcello Mastroianni had turned down the role of Carlucci, Billy Wilder tested both Nino Manfredi and Romolo Valli for the part. However, he worried that their accents would be too strong for American audiences and cast the New Zealand-born Clive Revill after the actor (whom he had recently directed in The Private Life of Sherlock Holmes (1970)) happened to call him on the telephone. Revill had played a Russian in his earlier Wilder film and had also played an Italian quite recently in The Assassination Bureau (1969); Wilder recalled his facility for languages and his skill at accents (he had also played both an Arab and a Scot in Modesty Blaise (1966), an American in A Fine Madness (1966) and another Russian in both Fathom (1967) and The Shoes of the Fisherman (1968) in the recent past) and hired him at once.
During a 2019 interview, Juliet Mills said she saw Jack Lemmon do something to get ready for each scene during this movie that she liked so much she started doing it too and still does. Before a take he used to rub his hands together repeatedly, very fast and say "Magic Time! Magic Time!" and that would help him focus the Adrenalin.