James Brolin's brother Brian Bruderlin was actually used for the image of the bearded man seen appearing from the 'red room' in the cellar (who is supposed to be Ronald DeFeo). The studio wanted someone who bore a close resemblance to Brolin and discovered he had a brother who shared a strong resemblance to the star. Brolin's brother was fitted with a fake beard for the part.
Because the movie was made on a relatively modest budget, James Brolin took less money up front but with a promise of 10% of the gross sales after its release. After the movie became an unexpected blockbuster (at that time it was in the top ten of all time), he eventually received about $17 million. If adjusted for inflation that would be equivalent to a little over $63.5 million in 2021 dollars.
(at around 20 mins) While washing the dishes, Margot Kidder's character is heard humming the love theme from Superman (1978) which she starred in as Lois Lane.
Due to all the unwanted fame the book and film had brought upon the real house in Amityville, the current owners have replaced the "evil eyes" windows with normal rectangle-shaped windows.