I Married A Monster From Outer Space (1958)

Starring Tom Tryon, Gloria Talbott, Peter Baldwin, Robert Ivers, Chuck Wassil
Directed by Gene Fowler Jr.

(actor & director credits courtesy IMDB.com)

On the eve of his wedding, an insurance salesman is secretly replaced by an alien being using his body, but before long his new bride begins to suspect he's an imposter. 

A distinctive and memorable science fiction drama, the picture has a well-written screenplay that slowly builds suspense as Talbott sees more and more evidence Tryon isn't the man she fell in love with.  It also has wonderful special effects for the era, courtesy of Universal veteran John P. Fulton, with luminous aliens, clouds of billowing smoke that envelop their victims, and atomizing ray gun blasts.  Talbott makes a lovely heroine, Tryon gives a stiff performance befitting a stranger in a strange body ignorant of human emotions, and there's good support from John Eldredge as the town's police chief and Ken Lynch as it's doctor.  The music and cinematography also contribute to the film's dark tone, creating a frightening tale of paranoia.