David Wood of Acts17 Apologetics has shared this excellent video of Greg Koukl of Stand to Reason explaining "The Columbo Tactic." I can tell you from experience that this is one of the most effective conversation strategies I have ever learned. Furthermore, Koukl's book Tactics transformed how I share my faith with others. I highly recommend both the video and the book!
Stand to Reason describes "The Columbo Tactic" as follows:
'Columbo' is most powerful if you have a game plan for the conversation. Generally when I ask a question I have a goal in mind. I’m alerted to some weakness, flaw, or contradiction in another’s view that I want to expose in a disarming way.
Other times the question is an open-ended 'What do you mean by that?' delivered in a mild, genuinely inquisitive fashion. The general topic can be anything broadly related to spiritual things. Then begin to probe with questions, gently guiding the conversation in a more spiritually productive direction.
The follow-up question, 'How did you come to that conclusion?,' graciously assumes the non-Christian has reasons for her view and is not just emoting. It gives her a chance to express her rationale (if she has one), giving you more material to work with.
Occasionally someone will quip, 'I don’t have any reasons; I just believe it,' to which I ask, 'Why would you believe something when you have no reason to think it’s true?' This is a genuine—and very appropriate—question. And it’s simple.
You may not always have an answer, but you can always ask a question, especially a well-placed one.
That’s the value of the Columbo tactic."
Courage and Godspeed,
Video: God of the Gaps? by Greg Koukl
"Taking the Roof Off" with Homer Simpson