Billy Wilder wanted the hero's father's long-running affair to have been with a male bellhop at the hotel; studio executives dissuaded him.
Billy Wilder originally designed the part of Carlucci for Marcello Mastroianni and hoped to get Walter Matthau to do a cameo performance as Blodgett.
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.
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.
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.
Juliet Mills gained 25 pounds within 6 weeks to take on the role of Pamela Piggott.
Avanti! marked the 5th of 7 collaborations between Jack Lemmon and director Billy Wilder, following Some Like It Hot (1959), The Apartment (1960), Irma la Douce (1963), and The Fortune Cookie (1966). The Front Page (1974) followed, and their last film together was Buddy Buddy (1981).
When recording the eulogy, Wendell says "The evil that men do", and then interrupts himself. This is a quotation from Shakespeare's "Julius Caesar" (Act 3, Scene II). The full line is "The evil that men do lives after them; The good is oft interred with their bones".
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.
At one point, Jack Lemmon says "What we have here is a failure of communication". This is a paraphrase of Strother Martin's famous line, "What we have here is failure to communicate" from Cool Hand Luke (1967) (1967). Jack Lemmon was a producer on "Cool Hand Luke".
Like many of the comedies of Billy Wilder, Avanti! deals with matters relating to marital infidelity. It's a topic that Wilder covered in such films as The Apartment, The Seven-Year Itch, and Kiss Me, Stupid. And yet Avanti! failed to garner much controversy, unlike Kiss Me, Stupid, for its depiction of marital infidelity, likely due to the nature of Wilder's classical style and the changing social norms in the intervening eight years between films. "It seemed so tame in a way," Mills said in 2017 of the film's handling of its characters' love affair, even with the nudity. "There were no great love scenes. It was a love story. It was more a romance and a romantic film to me than anything. There certainly wasn't any controversy about it, the adultery and all that. I think the fact that it was a romantic comedy. Jack Lemmon and Billy Wilder could really get away with anything, I think."
Juliet Mills said in a 2017 interview that while shooting, she was able to enjoy all of the delicacies that Italy had to offer. "All the time I was supposed to be eating and keeping my weight on. So [Billy Wilder] would feed me all the time and take me out to dinner every night with his wife and Jack Lemmon. I just had a wonderful time, a happy wonderful experience. I just had to have three big meals a day and finish up with a big bowl of ice cream," she said with a laugh.
The Helicopter landing place at the end of the movie is not at Ischia, but at neighbor island of Capri, the nearby lighthouse is the Faro De Carena. The "H" for Helicopter landing was simply painted on a parking ground.
Billy Wilder approached Lynn Redgrave to play Pamela Piggott, but she balked at having to put on weight for the part (Redgrave had been successful in losing a significant amount of weight).