The most terrifying element is often the unexpected. For example, when the interview setting is normal at first but then something really strange happens like sudden noises or odd behavior from the interviewers.
I think the power dynamic in horror interview stories is a terrifying element. Usually, in an interview, the interviewer has the power, but in these horror scenarios, it's taken to an extreme. They can create a situation where the interviewee feels completely powerless. For example, if the interviewers start to play mind games, like giving conflicting instructions or not reacting in a normal way to the interviewee's answers. This can make the interviewee feel like they are in a nightmare situation during the interview.
The sense of being judged in a very abnormal and uncomfortable way can be terrifying. In horror interview stories, sometimes the interviewers act as if they are not interested in the candidate's skills but in something else, like scaring or intimidating them. It makes the interviewee feel vulnerable. For instance, when they ask inappropriate questions not related to the job just to make the interviewee squirm. Another aspect is the environment. If it's a spooky place like an old building with strange sounds and bad lighting, it adds to the horror.