The funeral of Magistrate Simon Cordier (Vincent Price) is a grim event even for an occasion such as that, for it is laced with contempt for the deceased. Nevertheless, his last wishes are to be granted, and the mourners go back to hear a reading of his short diary of his last few days, which may shed some light into the out of character and bizarre manner he behaved during that time. He was a murderer at the point he died, and that crime appeared to have been triggered by his visit in a professional capacity to another killer, one who had put a number of people to death without any reason. But as those notes reveal, it was what Cordier heard during that fateful visit that pushed him over the edge.
Come 1963 and Vincent Price was becoming the face of American horror movies thanks to his Roger Corman adaptations of Edgar Allan Poe stories, but other producers wanted to get in on the act and couldn't understandably use Poe, so they cast around for other classic works of chiller fiction to bring to the screen, and in this case it was Robert E. Kent who was the driving force. Kent had made his money in B-movies, so for him Diary of a Madman represented a move towards respectability, though alas with himself on script duties he revealed himself to be no Richard Matheson as what he concocted was a rather dry and plodding variation on bits and pieces of Guy de Maupassant stories.
At least for the first hour there was very little happening save for a small selection of actors exchanging lines in oak-panelled rooms, the cheapness of the sets rather obvious, so presumably Price had taken most of the budget. That said, the presence of Nancy Kovack was not to be sneezed at, as she was an attractive addition to any cast, here seen in the heyday of her twenty year acting career, though most of that was in television rather than film. Her best known work in this vein was Jason and the Argonauts around the same time, which lent her considerably more screen immortality thanks to constant repeats on television somewhere in the world than anything she did here, and it should be noted she was not in the plot for the duration.
You can probably work out why that is from about a minute into her initial appearance, when she is introduced as a model for Cordier to indulge in his hobby of sculpture. You had to assume the subject of art was what made this project a shade more enticing for Price, as he was an avid collector, though the results of his character's endeavours would be unlikely to get into any gallery as he tries to capture Kovack's Odette in clay, and that "gay, yet strangely enigmatic" cheeriness she possesses. Good luck trying to convey that, Simon, as what it amounts to is a bust of a smiling Odette, but just as you're losing interest Kent thought up a more arresting use for the clay and offered one of the few jolts in the movie.
That last half hour was considerably more lively than the first two thirds of the story as Cordier has become seized by the power of The Horla (voiced by Joseph Ruskin), an invisible presence that has no other purpose but to drive men insane, and murderously so with it. Whenever the reluctant Cordier attempts to wrestle with his mind's unwelcome visitor, his eyes glow blue, i.e. director Reginald Le Borg shone a bar of light on the middle of his face, and soon all hell was breaking loose. Well, up to a point, as even with this theme it was still fairly restrained in what it was showing, relying on the frisson this supernatural presence could dominate the innocent into acting out of character in extreme ways. Though it didn't appear to be a metaphor, it was more or less what it said it was, which left a flimsiness to the way the crimes and their eventual cessation played out, as if Kent was leaning on the classic text without giving much further thought to his own work's implications. Besides, when you know from the start everyone who has survived, tension was lacking. Music by Richard LaSalle.