What type of questions should I ask when interviewing candidates?

The purpose of the interview is to determine if the candidate has all the skills, experience and abilities to perform to the level that you require for the role.  The questions therefore need to be designed to extract information from the candidate that will reveal his or her true potential.

By Googling interview questions you will come across thousands of questions many of which are really not helpful.  You will also come across behavioural style question structures such as the STAR process as well as Scenario questions which we often use.

One of the better questioning techniques was developed by Lou Adler who focuses on Performance Based Hiring.
He has only two principal questions and then drills down on the specific answers of each to reveal all the details of the person’s achievements and the way he thinks and conducts himself in the work environment.
  
The first question is to describe a significant accomplishment related to an actual performance objective.  The purpose is to relate this as closely as one can to the actual performance objectives of the role.

Once the accomplishment has been stated, questions are asked around role, responsibilities, issues and actions taken.  

Here are a few examples:

• Tell me about the project or plan?
• What was your role?
• What were the results?
• When was it?
• Who else was involved?
• What were the biggest obstacles?
• What were the critical decisions?
• What initiatives do you take?
• What changes did you make?
• What problems did you have to overcome?
• What resistance did you have and how did you deal with it?
• If you had to do it again what would you change?
• What was the work environment/culture like and how did it help or otherwise?
• What mistakes did you make?
• What did you learn and what recognition did you give/receive?

The idea is to progressively dig into the details to reveal more about the person you're dealing with. 

The second question is all about problem solving.  
The person is asked about how he or she would approach the problem of achieving one or more of the target outcomes of the new role.  Here, you are looking for thinking style rather than complete answers.