If you were interviewing someone for this job, what one question would you be sure to ask them?

Your answer to this question tells the interviewer something about how you view the world. What is more important for judging a person's fitness for a job - his specific technical knowledge or his general behavior and personality? Either one is a possibility. You may view a complete command of job-related technical detail as most important. No one can say this is wrong, but then you have to be prepared to deal with a wide range of personalities, some quite difficult. If you choose behavior/personality as the best differentiator between potential success and failure in a job, then you may need to be prepared to deal with a wider range of technical capability or knowledge.