According to an article in "Starlog" magazine it took 11 days to stop-motion animate the segment where all the heads appear on the Loch Ness monster.
Tony Randall shaved his head for the role of Dr. Lao. It also made it easier to apply the make-up for the different characters he played. The studio publicity department wanted to photograph Randall getting his head shaved but arrived at the barber too late. They had the make-up artist glue hair back on Randall's head so Randall could pose for photos while getting the hair cut off again.
Although it is now regarded as a classic fantasy film, this was a box office disappointment when it was first released. It caused a four-year gap before George Pal had his next film in theatres. It also marked that last time that Pal would direct.
William Tuttle received an Honorary Academy Award for his make-up work on this movie. It was the first of two Honorary Oscars awarded for make-up - the other one being John Chambers for Planet of the Apes (1968). It was not until 1982 that the Academy of Motion Picture Arts and Sciences decided to create a Best Make-Up category; An American Werewolf in London (1981) was the first film to win the award.
According to a story that appeared in "Life" magazine Tony Randall had his head shaved for the part of Dr. Lao. When it was decided to have him appear in the audience as himself during the second show they could not find a wig that looked like his natural hair so they took a woman's wig and cut and styled it enough to get by for the few seconds he would be on screen.
If you look closely at the three windup toys on the piano at the entry to the circus (33 minutes into the movie), one is Fred Flintstone who would not exist for another 60 years after the era in the movie.
The "Fall of the City" spectacular that Dr. Lao presents as the grand finale of his circus contains much footage from an earlier George Pal production, "Atlantis, The Lost Continent" (1961)
Before it became apparent the film was not a hit, George Pal told press he was planning a sequel with Tony Randall recreating his role.
Oscar-winning soundtrack composer Leigh Harline worked for Disney from 1932-41 and wrote the music for some of the best-known Disney film songs, including "When You Wish Upon a Star", "I'm Wishing," "Whistle While You Work," "Heigh Ho," and "Some Day My Prince Will Come."
The beginning and ending shots of the movie perfectly exemplify the "shooting-out-of-sequence" practice in film-making. You'll notice that the same exact camera setup is used for both shots, with Tony wearing the exact same costume. He simply rides toward the camera for the beginning of the story, and away from the camera for the end. Even the shadows from the sun are in the same alignment.
According to Ben Mankiewicz on Turner Classic Movies, Tony Randall only played six of the seven faces. The Abominable Snowman was actually played by George Pal's son, Peter Palmer, that played that role, "but George understood show business and declared that it was Randall playing the beast, to make the septet complete."
According to a July 1962 LA Times item, Laurence Harvey was set to play the Tony Randall role.
Tony Randall related on a 1963 episode of Password (1961) that his next film was called "The Circus of Dr. Lau," changed at some point before release to its eventual title.
At 38:55 sharp-eyed viewers will spot the Wicked Witch of the West's hourglass among the artifacts decorating Dr. Lao's circus tent. It's hanging on a post beside actress Lee Patrick and was most likely borrowed from MGM's prop warehouse.
Tony Randall and Barbara Eden also starred together that same year in "The Brass Bottle", a film which was a major influence on Barbara Eden's "I Dream of Jeannie" sitcom, that would premier the following year.
The Kate / Medusa-victim character was played by veteran actress Minerva Urecal who, along with Marjorie Main, was the inspiration for the voice of Witch Hazel in the Warner Bros. Looney Tunes cartoons.
Child actor Kevin Tate later appeared with Barbara Eden on I Dream of Jeannie (1965) in I Dream of Jeannie: What's New, Poodle Dog? (1966).