Why it is important to ask difficult questions at an interview

The essential aim of a job interview is to find out what kind of person a candidate is, what their personality is and how they come across face to face. If only facts, experience and data mattered, recruitment would be done solely by looking at CVs. And because you get to know a person best in extreme and non-standard situations, it is necessary to be able to ask non-standard and difficult questions during interviews. What types of questions should you ask in order to learn the most about a candidate?