How we change what others think, feel, believe and do
Manipulate the situation into one of informality and then, when their guard is down, pop in the key question.
Make it appear that they are not being interrogated.
Let them think that the interrogation is over and that they are being released. Then ask the question on the way out.
Let them think that they have already given you all the information you need.
A manager takes a person being recruited out to lunch after the interview, where some key questions are dropped into the conversation.
A seemingly-unintelligent guard offers the prisoner sympathy after the interrogator has left. They fall into conversation, in which the prisoner lets slip the information they have been bottling up.
That's it. We're done. Let me open the door for you. Oh, by the way, was that John who was with you at the party last week?
Done well, the person will not even know that they have given you the information you are seeking.
Columbo, the TV detective, was a master of creating informality by appearing casual and incompetent. When the other person had discounted him as a non-threat, he then asked the key question that got him the information he wanted.