Child actress Toni Fernandez who shares a brief scene with Sara Montiel at the beginning of the film is actually Sara's niece in real life.
One of the characters in this film is called the Grand Duke Vladymir of Russia (played by Alfredo Mayo as required by the script: a drunken aristocrat who is after Maria Lujan (Sara Montiel). It so happened that there was in real life a Russian Grand Duke by that name and he sued the film for portraying him in such an unflattering manner. The problem was resolved when Cifesa (the film's distributor) agreed to pay the indignant Duke an undisclosed large sum of money and removed from the soundtrack any reference to Vladymir of Russia. Since then all you hear in the movie is references to "Grand Duke" but no name.
When the original script of El Ultimo Cuple was submitted to the censors, as required by law in General Franco's regime, it was rejected. The story described the Paca character as a typically aggressive stage mother pushing her daughter Maria into the morally challenged "cuple" singing career. Censors saw it as immoral behavior for a Spanish mother in Catholic Spain and they asked the writers to change the character from mother to aunt. The change was made but the censors thought it over and decided that it was also improper behavior for an aunt. The writers came up with the perfect solution by adding a two-line dialog where Maria's boyfriend tells her: "Why care about her? She is not even a blood relation of yours." To which Maria replies: "It doesn't matter. She raised me and sacrificed for me. She loves me in her own way." The censors were happy this time and approved the script.
It was originally planned that Sara Montiel would lip-sync the songs previously recorded by another singer. However at the recording studio, the hired singer demanded to be paid in advance for her services and producer-director Juan de Orduña could not come up with the cash. Sara was summoned to the studio and recorded all the songs in one afternoon. Sara's voice and singing style ended up being the main attraction of the film.