The real Matthew Hopkins was only in his mid 20s in 1645 and died before he was 30. Vincent Price's character is well into middle age, 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 about 500 people in total were executed for witchcraft in England between the late 15th and late 18th centuries, this means Hopkins was responsible for two-thirds of these executions during a period of three years.
On the first day of filming Vincent Price fell from his horse. Director Michael Reeves subsequently refused to see him hoping that angering Price would help the actor make his character more fierce.
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, heavily re-writing the script to better fit Price's persona, making Hopkins more imposing and menacing instead of pathetic and effeminate as originally intended. Upon meeting Price for the first time, Reeves told the star, "I didn't want you and I still don't want you; but I'm stuck with you." Price however, did not take this lying down, and after one of their arguments told the director, "Young man, I have made 84 movies. How many have you made?" Reeves, who up to that point had only made the critically reviled She Beast (1966), and The Sorcerers (1967), responded defiantly, "Two good ones!" causing Price to laugh loudly.
Near the end of filming the tension between Price and Reeves was much in evidence, but both men came to respect each other begrudgingly as they agreed the final product was a good one.