The real Martha Beck was of a very different stature to that of actress Salma Hayek and at the time of her death weighed well over 200 lbs. On her execution day, it was discovered that she would not fit into the electric chair, so was seated on the arm rests when the fatal shock was administered.
The director's grandfather was Elmer C. Robinson, the cop who investigated the Lonely Hearts murders. The director dedicated the film to his grandfather.
Jared Leto shaved the front of his head and the bald patch to get the effect of the real Raymond. He even stated he plucked his hair on his head to add to the regimen of shaving.
While the real Raymond was Spanish and the real Martha was white their actors who play them is Jared who is white and Salma is Spanish.
Salma Hayek wore blue contacts throughout the film because the real Martha had blue eyes but Salma has deep brown eyes.