A lot of the reactions of terror from the actors were real as they suffered from the claustrophobic confines of the caves they were shooting in.
The pub featured in the film - the Union Inn - also provided the catering.
The Irish priest is named Mark Amadin. Throughout the movie, he refuses to believe that there are supernatural forces at work. "Amadán" in Irish means "fool".
The film was only completed days before its premiere at the 2013 Frightfest in London.
Leading British film critic Mark Kermode is a great fan of the film and the horror genre in itself. The first time he saw the film, he was half tempted to walk out of the cinema, he was so unsettled.