The real Matthew Hopkins was only in his mid 20s in 1645 and died before he was 30. Vincent Price's character is middle-aged, like the actor himself. Hopkins and Stearne executed more than 300 people, mainly women, during their two or three years of '"witch hunting". Considering that 500 people in total were executed for witchcraft in England between the late 15th and late 18th centuries, it means that Hopkins was responsible for two-thirds of these executions during a period of three years.

Vincent Price regarded his performance here as the finest of his horror movie career.

Director Michael Reeves wanted Donald Pleasence to play Matthew Hopkins, but American-International Pictures, the American distributor and co-financier of the film, insisted that Vincent Price play the title character, and Reeves grudgingly accepted.

On the first day of filming, Vincent Price fell from his horse. Director Michael Reeves refused to see him, hoping that angering Price would help the actor make his character more fierce.

The film takes place in 1645.

Paul Ferris, who wrote the film's dramatic music score, also acted in the movie under the pseudonym Morris Jar as an homage to film composer Maurice Jarre.

The interiors were filmed in two specially converted aircraft hangars near Bury St. Edmunds in Suffolk, which were leased for £1,500; this cost-measure resulted in much of the dialogue having to be re-recorded later, because the tin roofs of the hangars caused an echo. The exterior shots range from the Dunwich coast (for the scene with the fisherman) to Langley Park outside London (for the scene where Stearne escapes capture). The tracking shot of the ambush after the opening credits was filmed at Black Park in southeast Buckinghamshire, a location frequently used by Hammer Film Productions. Lavenham Square (in Lavenham, Suffolk), site of the witch-burning scene, was the real Lavenham Market Square; the crew lowered TV antennas and telephone wires and producer Philip Waddilove hired a cherry picker from a local utility company for £10, because the unit couldn't afford a camera crane. The countryside vistas seen in the chase scenes on horseback were shot on the Stanford Battle Area near Thetford, Norfolk--the producer, through connections with the government, was able to lease parts of the area. The church used in the film is St. John The Evangelist in Rushford in Norfolk. The moat drowning and hanging scenes were filmed at Kentwell Hall, in Long Melford. The climax of the film was shot at Orford Castle, on the coast of East Anglia, which is an English Heritage property. Filming wrapped as scheduled on 13 November 1967. The production went relatively smoothly, except for the unrelentingly antagonistic relationship that developed between director Michael Reeves and Vincent Price. Reeves kept it no secret from everyone associated with the production that the American actor was not his choice for the role, and the director's comments had reached the actor back in the US. Reeves refused Price the courtesy of meeting him at London's Heathrow Airport when he arrived in England, a "deliberate snub calculated to offend both Price and AIP". "Take me to your goddamn young genius," Price reportedly said to co-producer Philip Waddilove, who greeted the actor at the airport instead of Reeves. When Price went on location and met Reeves for the first time, the young director told the actor, "I didn't want you, and I still don't want you, but I'm stuck with you!". According to Kim Newman in his book "Nightmare Movies", when Reeves made a suggestion on the set, Price objected and told the director, "I've made 87 films. What have you done?" And Reeves responded, "I've made three good ones". Price later recalled, "Reeves hated me . . . He didn't want me at all for the part. I didn't like him, either. It was one of the first times in my life that I've been in a picture where the director and I just clashed." Price felt that all the actors on the set had a difficult time with the director, explaining: "Michael Reeves could not communicate with actors. He would stop me and say, 'Don't move your head like that.' And I would say, 'Like what? What do you mean?' He'd say, 'There--you're doing it again. Don't do that'." Price reportedly became so upset with Reeves that he refused to watch the film's dailies.

A completely new score was written for the US video release by Kendall Schmidt.

Even the truncated version was met with considerable controversy by UK film critics. Dilys Powell in "The Sunday Times" complained, " . . . 17th-century hanging, burning, raping, screaming and Vincent Price as England's prize torture-overseer. Peculiarly nauseating". "The Guardian" felt the film was filled with "gratuitous sadism". Margaret Hinxman of "The Sunday Telegraph" dismissed it as a "sadistic extravaganza". Nonetheless, several critics felt the film was worth accolades. John Russell Taylor in the "London Times Saturday Review" said the film " . . . is quite happily and deliberately a horror film: that is to say, it has no particular pretensions to being anything else . . . There is much in it which would win Michael Reeves an important reputation if he were dealing with some more pretentious, but fundamentally no more serious subject . . . Mr. Reeves is no longer merely promising. He already has real achievements behind him: not merely good horror films, but good films, period". Films and Filming noted that the film "has no explicit 'message', but it does say something about the springs of despair and it says it forcefully. It is a very frightening film . . . Matthew Hopkins is the best of Price's recent performances. [The film] is emphatically not a horror film; it is, however, a very horrifying one . . . ". Monthly Film Bulletin observed, "Not since Peeping Tom (1960) has a film aroused such an outcry about nastiness and gratuitous violence as this one . . . the tone of the film is oddly muted, with torture and death in plenty, but viewed matter-of-factly and without stress . . . Throughout the whole film there is a vivid sense of a time out of joint, which comes as much from the stray groups of soldiers who skirmish against unseen attackers in the woods or hang wearily about by the wayside waiting for battle to commence, as from the bloody crimes committed in the name of religion by Matthew Hopkins".

During his prayer, Richard swears an oath to God that he will not stop from the pursuit of Priest Lowes' killers until they're ready to stand before Him. However, the Bible states in James 5:12, "But above all things, my brethren, swear not, neither by heaven, neither by the earth, neither by any other oath".

Robert Russell was re-voiced by Bernard Kay.

Although there is a church in Brandeston Suffolk,the actual church used in the film (the diocese of John Lowes) was St John's,Rushford near Thetford in Norfolk.

Closing credits: All characters in this film are fictitious. Any resemblance to any person living or dead is coincidental.