Elvis Presley later said about the film: "I wasn't exactly a James Bond in this movie. But then, no one ever asked Sean Connery to sing a song while dodging bullets."
One lucky Elvis Presley fan bought a jacket as a gift to Elvis. Elvis told her to watch the movie carefully when it came out. When the film came out, she saw that the jacket was in the suitcase belonging to Elvis' character.
The film is set in Europe, but Elvis Presley never filmed outside Culver City, California. Location work in Belgium used body doubles Joe Clauwers for Elvis Presley and Polly Geerts for Annette Day. Elvis never traveled outside the U.S., for either movies or concerts, because his manager, Tom Parker, was actually not an American citizen--he was Dutch and technically was in the country illegally--and was afraid that if he left the U.S. he would not be allowed back in if it was discovered he was an "illegal alien".
The first and only film made by Annette Day. Producer Judd Bernard found her while he was with his wife shopping in London and entered an antique shop owned by her parents. He asked her to have lunch with him and talk about a part in a movie. Bernard, Day and Lee Marvin had lunch together in a hotel. Later Bernard went back to Hollywood, and soon Day got a message from him asking her to travel to Hollywood and make a screen test. She got a role in the movie.