This is one of these questions where the answer is not universal. There is not just one skill that would propel you to the UX design hall of fame. There are foundational skills that definitely help like being curious, conscientious, lifelong learning having empathy, being able to communicate clearly (through writing or speaking, aesthetics, being able to focus, being inquisitive etc.
Interviews have a lot of issues and they provide a great setup neither for the employer nor for the potential candidate. Another approach is to actually hire a shortlisted candidate to contribute on a real task under actual operational conditions. This will give a much better idea over a longer timespan and reveal whether the candidate is a correct fit.
There is no specific UX principle as long as we empathize with the user and apply design thinking. The challenge of UX for the interview is the complex problem which you solved in any design project.
However, there are other lots of interview questions which can also be looked upon for cracking a UX job.
