Screenwriter Leder was originally set to direct the film, but being American, was unable to obtain a British work permit in time, so Crabtree replaced him as director. Thompson later said that when the director showed up on the first day of shooting and looked at the script, Crabtree claimed it was not the film he had been hired to direct, as he did not do "monster" films. After a heated argument with the producers, Crabtree left the set and did not show up for several days. In the interim, Thompson himself directed the film.