Start with broad general questions and listen to the candidate’s answer. Ask careful follow-up questions to probe more deeply, whenever you find you lack complete answers (the who, what, where, when, and hows?).
Careful means don’t ask leading or judgmental questions; rather than, “You did what? Why on earth would you have done that?” ask, “And how did you decide to deal with that issue?” or, “How did you find yourself reacting when you discovered that?” Pay attention to your tone of voice here, too.
The same words said with interested curiosity sound very different when said with a judgmental or doubting tone.