Or should I say, Ray Harryhausen's creature IS the film. Compared to the creature, the human habitants here don't have a chance to emote and are only what one might charitably call "adequate" to the task.
WILLIAM HOPPER lacks the energy and personality he put into playing Paul Drake in all those "Perry Mason" TV movies. He appears to be almost immobilized and never changes his expression, even when the creature is doing maximum damage to the surroundings. JOAN TAYLOR is quite forgettable as "the girl", a woman who tells Hopper that she's "almost a doctor", when introducing herself. She's almost an actress too.
But nobody is going to give a whit about the acting in this sort of thing, so I'm happy to report that the frightful looking bit of slime that turns into a fast-growing creature with a human torso and scaly skin is extremely well animated by Harryhausen, who creates some very realistic looking fight scenes and displays of temper as the creature goes on the loose. There's an awful lot of screaming going on, both from the creature and the spectators who run into him.
Helpful too is the fact that this was filmed in Italy, where the action takes place, and there's a terrific climax shot in the Roman Coliseum where the creature finally meets his fate.
For the child in all of us, this is terrific fun--but just don't expect much from the human cast. The story is absurd and follows the usual sci-fi pattern of keeping you glued to your seat just to see how it all turns out.