Dr. Charles Marlowe and Mr. Edward Blake, the main characters in this movie, are not fooling anyone. This Amicus film is really an adaption of Strange Case of Dr. Jekyll and Mr. Hyde and has Christopher Lee in the main role and Peter Cushing as his lawyer Utterson, who thinks that Marlowe and Blake are two different people.
This is the first movie that Stephen Weeks directed. He’d go on to make Sword of the Valiant for Cannon in the 80s. The script comes from Amicus head Milton Subotsky.
Originally intended to be released in 3D, the film used the Pulfrich effect — in which the “lateral motion of an object in the field of view is interpreted by the visual cortex as having a depth component, due to a relative difference in signal timings between the two eyes” or in short, the eye and mind are fooled into seeing depth where there is none — it seems like the foreground is always moving to the right and the background sliding to the left.
Peter Cushing has said that this was one of the least enjoyable movies he made, but I’ve heard that about several films.