Patrick Stewart shot one of his scenes as scripted with the dialogue line "He’s been bad". He pulled Tobe Hooper aside and whispered that maybe saying "He’s been naughty" would be better. Hooper agreed, and that's the version they went with for the film.
The scene where The Space Girl is speaking from inside a possessed Dr. Armstrong uses a verbal cadence and cuts between Stewart and Mathilda May to show their connection. They had struggled as to how to make that clear until Stewart came up with the cadence idea and went off with May to practice.
Part of the reason May got the role of the vampire was because, in addition to being comfortable with the nudity, May had trained as a ballerina, which allowed her to move in ways that made the vampire more creepy.
Henry Mancini had agreed to do the film, based on the original concept of a fifteen-minute essentially dialogue-free opening sequence, involving the discovery and exploration of the alien spacecraft, and the moving of the three aliens back to the Churchill, for which he composed a tonal "space ballet".
Steve Railsback signed on because he was eager to be in a science fiction film.
Box Office Bomb: The film cost $25 million to make and Cannon expected it to be a blockbuster. Final box office gross: $11 million.
Before being cast as Dr. Armstrong, Patrick Stewart was considered for Dr Leonard Bukovsky, Sir Percy Heseltine, Col. Colin Caine, Kelly and Lamson.
Frank Finlay was considered for Dr. Armstrong, Bukovsky and Sir Percy before being cast as Dr. Hans Fallada.
Michael Gothard was considered for Col. Caine before being cast as Bukovsky.
Peter Firth was considered for Roger Derebridge before being cast as Col. Caine.
Nicholas Ball was considered for Col. Caine, Kelly and Lamson before being cast as Derebridge.
John Hallam was considered for Bukovsky, Derebridge, Col. Caine and Kelly before being cast as Lamson.
Creator Backlash: Patrick Stewart describes the movie as "the nadir of his career" and, as stated above, took the job solely because he was in need of some cash. He did say, however, that Hooper was his favorite director to work with.
It is also one of multiple examples of the hubris of The Cannon Group — while they finally had some critical success with Runaway Train, which came out the same year. Their over-confidence in Lifeforce and other films that became box office bombs led the movie studio down a road to ruin, eventually closing up shop nine years later in 1995.
Brian Carroll, who plays one of the Churchill crew, was killed in a road accident, while the film was in post-production. In editing, most of his role ended up being cut, plus all of his lines were cut. In the end, his role in the film was reduced to being an extra.
Most of Nicholas Ball's performance ended up on the cutting room floor, according to an interview he gave on Wogan in 1985.
Despite being credited on the 101 minute cut, the following actors were deleted from this cut of the film: John Woodnutt, John Forbes-Robertson, and Russell Sommers. Plus Carl Rigg has all his lines removed from this version.
Development Hell: At some point, two TV series were announced based on the property: one by Ringleader Studio in 2013 and based on the book and another by Chiller TV in 2015 and based on the film. So far, both projects have failed to materialize.
Disowned Adaptation: After its release, Colin Wilson recalled that author John Fowles regarded the film adaptation of Fowles' own novel The Magus as the worst film adaptation of a novel ever. Wilson told Fowles there was now a worse one, the film adaptation of Space Vampires.
Tobe Hooper initially wanted to make Lifeforce a much sillier movie titled The Space Vampires, the same name as the book on which it is based. The execs forced him to make the story more serious, and switch to the present title.
When Tristar Pictures picked up the film for domestic distribution, they cut 15 minutes from the film and had some of Henry Mancini's score replaced with work by Michael Kamen. Due to the cuts, the US version is rather confusing at times. The original cut is included on the 2014 Blu-Ray release from Shout! Factory.
Troubled Production: The film went over-budget, leaving several and (allegedly vital) scenes to not be filmed because there wasn't any money left. This is reflected on the movie's rather disjointed nature that translated into mediocre reviews and poor box office performance. The failure of Lifeforce effectively killed Tobe Hooper's Hollywood career and contributed to Cannon Films going the way of the dodo in the early 90s.
Uncredited Role: Michael Armstrong and Olaf Pooley did uncredited work on the script.
Fallada and Bukovsky were both originally intended to be German, but were changed to British due to Frank Finlay and Michael Gothard acting as vocal coaches to Matilda May, who did not speak English.
The film was supposed to have a Tomato in the Mirror ending by having one of the protagonists revealed as a space vampire. It would have worked, too, if this didn't contradict the rest of the movie.
Michael Gothard was very close to being cast as Col. Colin Caine, and was eager to play the part after being relegated to smaller roles throughout his career. However, the role was given to Firth and Gothard was given Bukovsky. According to friends and family of Gothard, losing out on the part put Gothard into a deep depression.
Working Title: The Space Vampires, same as the book. Cannon changed it because they wanted it to be a blockbuster and didn't want it to sound like one of their exploitation films.
Go Nagai, highly prolific manga artist, Trope Codifier of Fanservice in anime and manga and creator of Mazinger Z, Devilman, Kekko Kamen and Cutey Honey among other, is said to have been greatly impressed with Mathilda May. Given the uncanny resemblance that Mathilda possesses to the heroines of Nagai's works, namely Sayaka Yumi from Mazinger Z and Jun Fudo from Devilman Lady and the fact she portraits an impossible powerful nude female that attacks men with reckless abandon, a very noticeable instance of Author Appeal in his works, one can see why.
Frank Finlay was made a Commander of the Order of the British Empire during the filming of this film.
Frank Finlay plays a character similar to Abraham Van Helsing. In the 1977 version of Count Dracula, Finlay did play the Van Helsing role.