The French-speaking cast struggled with the English dialogue, and some actors simply remembered their lines phonetically. The footage was then overdubbed by actual English speakers, who had to follow the odd speech rhythms of the actors.
The castle decorations weren't any additions from the art department, it was the castle's decoration. Its owner was a beer merchant.
The creator of the special effects was 18 when the film was made.
In the scene in which one of the grandmothers cuts the cake with her fingernail, it had to be done the first time since they only had one cake made. As in the scene in which one of the grandmothers, already transformed, goes through a painting from the 17th century. The director arranged for a painter to make him a free copy of the painting.
Despite the title, this movie is about demonic possession and does not involve the rabies virus. Furthermore, the two ladies in question are not grandmothers, but in fact aunts. So the actual title should rather have been "Demonic Aunts".