We examined the efficacy of a new approach to detect truths and lies in expressing opinions: the Devil's Advocate approach. Interviewees are first asked an opinion eliciting question that asks participants to argue in favour of their personal view. This is followed by a Devil's Advocate question that asks participants to argue against their personal view. People normally think more about reasons that support rather than oppose their opinion. Therefore we expected truth tellers to provide more information and shorter latency times in their responses to the opinion eliciting question than to the Devil's Advocate question. Liars are expected to reveal the opposite pattern as the Devil's Advocate question is more compatible with their beliefs than is the opinion eliciting question. In Experiment 1, we interviewed seventeen truth tellers and liars via the Devil's Advocate approach and measured the difference in number of words and latency times to the two questions. Our hypotheses were supported. In Experiment 2, 25 observers were shown these interviews, and made qualitative judgements about the statements. Truth tellers' opinion eliciting answers were seen as more immediate and plausible and revealed more emotional involvement than their Devil's Advocate answers. No clear differences emerged in liars' answers to the two types of question. We conclude that the Devil's Advocate approach is a promising lie detection approach that deserves attention in future research.