- Awesome, Dear Boy: According to Ken Russell, Vanessa Redgrave was having a lot of fun playing a sexually frustrated nun.
- Banned in China: The film didn't have a chance in Italy with its blasphemous content and was banned. Vanessa Redgrave and Oliver Reed were also threatened with three years in prison if they ever visited the country.
- Channel Hop: United Artists announced the film in August 1969, with Robert Sole to produce under a three picture deal with the studio, and Ken Russell to direct. Filming was set to start in May 1970. However, after United Artists executives read the screenplay, they "refused to touch it," abandoning plans to fund the production. At the time, production designer Derek Jarman had already constructed some of the sets for the film. It was subsequently acquired by Warner Bros., who signed on in March 1970 to distribute the film.
- Creator Backlash: Ken Russell was hugely annoyed at being forced to cut the scene of the rape of Christ, feeling it undermined the point of the story. But "short of burning the entire film, what else could I do?"
- Creator Breakdown: Ken Russell's marriage fell apart during the filming of this movie.
- The Danza: Subverted, possibly due to a goof. Judith Paris is credited as 'Sister Judith' but in the film she's called Sister Agnes.
- Dyeing for Your Art: Oliver Reed had his hair cut off, and shaved his beard and eyebrows for the climax. Especially notable because he grew the facial hair out to hide his scars.
- Enforced Method Acting: Vanessa Redgrave wore a prosthetic hump to help keep in character.
- Executive Meddling: Ken Russell cut the infamous "Rape of Christ" scene and original ending personally after showing a rough cut to John Trevelyan, the Secretary of the British Board of Film Censors, who said those scenes would be impossible to pass. Despite this the British Censors demanded further cuts of 89 seconds to make some of the sex and violence less explicit. They then proceeded to pass it with an X certificate to a storm of protest. The real damage was done by Warner Bros., who hacked the film to near incoherence. For example, any trace of pubic hair was removed, cutting 6 minutes to release it with an R rating. They then proceeded to get their hands on all copies of the British version and cut them to match the R version, preventing the film being seen in anything close to its original form for decades. For years the cut footage was presumed to be lost forever, until most of it was ultimately recovered; but while the "rape of Christ" sequences have been restored, the original ending (with Sister Jeanne using the charred bone of Grandier as a dildo) has yet to be found and possibly remains lost forever, though this might be for the best. The director's cut is occasionally seen at film festivals in the UK but Warner Bros. have repeatedly denied requests for it to be released on DVD.
- Follow the Leader: It's theorised that this was following the lead of Witchfinder General, being Very Loosely Based on a True Story and depicting explicit violence in a tale of religious hypocrisy. It's also said that The Devils paved the way for nunsploitation films of the 1970s.
- Hostility on the Set: Throughout the shoot, Ken Russell and Oliver Reed clashed frequently, and by the time principal photography had finished, the two were hardly on speaking terms.
- Jossed: Ken Russell denies the story that a technician misunderstood a direction from him and detonated explosives too soon for the final scene — saying that he himself pressed the button without telling the camera team.
- Keep Circulating the Tapes:
- Still not available on DVD in the US; the main sources for obtaining the film either are the US edit VHS tape, importing the partially restored BFI DVD from the UK, or pirating it online. It has shown up on streaming services, as detailed below.
- Part of the problem is that Warner Bros. studio owns the US rights to the film and it has been stated that there are people in the studio who find the film to be blasphemous and not worth the shitstorm they fear would be unleashed if it ever saw the light of day in the US as far as the religious right's reaction. Furthermore, any release that the film has would be the heavily butchered US edit, as Warner Bros.' contract is for that version and not the recently "restored" version.
- Similarly the rights to the documentary on the making/restoration of the film Hell on Earth. It's been stated that Warner has no interest in the documentary whatsoever in the US.
- As of late 2017, a 109-minute "restored Director's Cut" began appearing on streaming services Shudder and FilmStruck only to be removed within months (in the case of FilmStruck, because the entire service was discontinued). Before it vanished, however, it was quickly ripped and pirated — possibly by those who suspected this very thing would happen.
- It was also briefly on FilmStruck successor The Criterion Channel around Halloween, 2019, but has since been removed. As of September 2020, its still on Shudder.
- No Export for You: The film has never been released on Blu-Ray or DVD in the United States (it was briefly available as a digital download, but it is no longer available). It was also briefly available to stream on Filmstruck, but the service itself was shut down in late November 2018.
- Production Posse: One of nine films Ken Russell made with Oliver Reed.
- Reality Subtext: Oliver Reed stated that the film's harshness was heavily influenced by The Troubles in Northern Ireland at the time.
- What Could Have Been:
- Ken Russell wanted Sister Jeanne to be the main focus character, covering the real woman's journey after Grandier was executed — wherein she became a national celebrity and convinced priests that she'd received the stigmata. He really wanted to include a scene depicting her death — after which she was beheaded and the head placed in a glass casket in her own convent.
- Twiggy and her manager/boyfriend Justin de Villeneuve were to cameo in the court scene. But once the nuns started stripping, the two walked off the set. They'd only filmed one shot.
- Spike Milligan was originally cast as Baron de Laubardemont but Ken Russell felt he couldn't convince in such a dark role, and recast him.
Trivia / The Devils