The Horror Of It All (1964)

Starring Pat Boone, Erica Rogers, Dennis Price, Andree Melly, Valentine Dyall
Directed by Terence Fisher
(actor & director credits courtesy IMDB.com)

A young American living in England visits his British future in-laws and discovers there may be a murderer among them, trying to kill off the rest of the family.

This is a fun little movie with actor/singer Pat Boone trying to find the murderer among a cast of macabre characters.  It's essentially a horror/comedy but affords Boone one musical number, singing "The Horror Of It All," to describe the sinister goings on.  The most intriguing thing about the movie is that although the story is credited to Ray Russell, it is very similar to the J.B. Priestly novel "The Old Dark House," which was filmed before in 1932 and 1963.  Interestingly enough the 1963 version was produced by William Castle who directed Russell's script for Mr. Sardonicus in 1961.  I had a fun time with this movie, with its quality British supporting cast, jazzy music score, and Boone's cute romance with leading lady Erica Rogers.