We examined a common, but understudied phenomenon: Assessing interviewees' truthfulness when they attempt to conceal their knowledge about another person. We argue that this should be mentally taxing because truthful answers are activated automatically, and hence, need to be suppressed when liars conceal their knowledge. Participants were shown three photographs of three females, only one of whom was known by the participants. The participants were asked questions about each female depicted in the photograph. Truth tellers were instructed to give the correct answers when discussing the female they know whereas liars were asked to conceal their knowledge about this female. Independent observer ratings revealed that liars appeared to be thinking the hardest when discussing the female they know, whereas truth tellers appeared to be thinking the hardest when discussing the females they did not know. Truth tellers looked the most at the photographs of the unknown females, whereas liars' gaze did not differ across photographs. Finally, liars' answers about the female they know contained substantial truthful elements. In summary, truth tellers' and liars' responses differed substantially and those different responses are valuable cues to detect deceit.