Dominic Perez made this film when he was 40 years old. He had wanted to make a movie for years but had never gotten around to it. In 2008, the recession happened and the investment bank he worked at cut his entire division of 2,000 people. Some of the people fired had worked there for 20 years, and Dominic saw them weeping as they collected their belongings and were escorted out by security. Perez was transferred to another division, but was told that he would also be let go in three months. With this realization, Perez decided that he would make the film.
Tanya's illness was not in the script. Torrey Weiss genuinely was sick during filming, so Perez wrote that into the story. The snowstorm also was not in the script, as it was a freak storm that hit New York in the middle of filming.
Dominic Perez spent four months writing and rewriting the script. The film itself was shot in one week and edited in three months.
The film was inspired by a real life incident that happened to director Dominic Perez when he was a child. Perez was sleeping over at a friend's house while his mother was at work and there was a soft knock at the door. They asked who it was but there was no answer. The peephole was broken so they couldn't see who it was, but they could see under the door that the person wore black boots. 30 seconds later, the person knocked again, and this went on for ten minutes. They never opened the door or found out who was on the other side. Dominic's goal with Evil Things was to take the sheer terror inspired by that real event and re-create it for the characters in his movie. The young characters in Evil Things dare to open that door and make contact with what is on the other side.