The green monster footage and fairground destruction were re-shoot material filmed during several very rainy nights in Malibu. Gore video guru Larry Wessel has a bit part murdering a Humanoid in the crowd scenes.
As the film was being finished up, producer Roger Corman felt that the picture needed more sex, and ordered scenes shot that showed the "humanoids" attacking - and, of course, ripping the clothes off of - other nubile young women. Director Barbara Peeters refused to shoot the scenes, protesting that they were inserted purely to show gratuitous nudity. Corman fired her and hired another director to shoot the additional footage. However, most of the re-shot Humanoid attack scenes were deleted from the final print of the movie.
Director Barbara Peeters once said that the "humanoids" were originally suppose to be played by the film's stunt men. Unfortunately, the stunt men found the monster suits to be too "goofy-looking" and refused to wear them. Additional actors had to be hired to perform as the monsters.
There are only three Humanoid creatures seen together on-screen in the same frame throughout the movie. The production really only had one fully functioning Humanoid costume and two others that could only be shot from certain angles because they weren't as convincing. Use of such angles, as well as the editing, would help to create the illusion that there were much more Humanoids than just three for the climactic carnival massacre.
Actress Ann Turkel once said why she chose to do this film: "It was an intelligent suspenseful science-fiction story with a basis in fact and no sex". However, with the filming of additional footage, the sex content changed.
According to Movie News (Australia) magazine, after her discovery of the explicit additional footage, "Ann Turkel was so outraged that she asked Hollywood's Screen Actors' Guild to try and stop 'Monster' [Humanoids from the Deep] from being released".
The production was originally filmed under the title "Beneath the Darkness" in the hopes that it would infer a classier type of story and therefore attract the kind of big name cast the filmmakers wanted.
The film was made and released about sixteen years before it was re-made for television with Humanoids from the Deep (1996) but the second time the levels of sex and violence for the remake were greatly toned down.
Showbusiness trade-paper Variety said of this movie in 1980 that "with Humanoids from the Deep (1980), Roger Corman comes full circle back to his very first film as a producer, Monster from the Ocean Floor (1954) . Despite costing 100 times as much, new pic has similar premise and same raison d'etre, that of pocketing a profit from drive-in dates" with the picture having "more nudity and gore than carried by any exploitationer in recent memory".
One of two Creature from the Black Lagoon (1954) type films featuring amphibious creatures released within a year of each other around 1979/80. The other movie was Screamers (1979) [The Island of the Fishmen].
This movie and Portrait of a Hitman (1979) were the first films that actress Ann Turkel made without on-and-off partner husband actor Richard Harris who Turkel had at the time just made three back-to-back consecutive pictures with.