John Cassavetes was part of the original production but he became ill and filming was delayed for three weeks. In the end, he was replaced by Rip Torn. This, at least, was the story put out by the film's publicists at the time - however, later accounts tend simply to say that Cassavetes walked off the movie.
Sol Madrid (1968) was the last theatrically released movie Oscar® recipient Paul Lukas made, playing Mob "Capo Riccione." Born in Hungary, Lukas was established as a comedic actor by age 23. He journeyed to Hollywood in 1927 where he worked as an actor for the next 41 years. In 1944, he won an Oscar® for Best Actor in a Leading Role and was a Golden Globe Winner for Best Actor for the same role in Dashiell Hammett's Watch on the Rhine (1943) with Bette Davis playing his wife.
John Cassavetes, who left the production early, said of one of his co-stars "Ricardo Montalban is to improvisational acting what Mount Rushmore is to animation."
John Cassavetes, who was originally set to star in this film in the Rip Torn role, specifically requested Stella Stevens for the role of Stacey. She was under contract to Columbia at the time and her schedule was adjusted to accommodate the loan out. She has said she was thrilled to work with William Tuttle (makeup), Sydney Guilaroff (hair) and Moss Mabry (wardrobe) again. She had previously worked with some or all of them on Advance to the Rear (1964) and The Courtship of Eddie's Father (1963) for MGM.
After the success of the James Bond film series, movies of the 1960's were crawling with super-spies and secret agents. It wouldn't be long before filmmakers stopped pitting the agents against unbelievable maniacs bent on world domination and turned them instead against more realistic criminals like drug czars or organized crime lords. One such film is Sol Madrid (1968), which was titled The Heroin Gang in England.