Popular ideas about lie detectors are dominated by the image of the polygraph machine. The iconic old suitcase polygraph is a vital part of North American film history and it still has not lost any of its visual power today. The imagined function of the polygraph as a lie detector is not just part of a vivid visual narrative, but has also influenced actual fields of research and practice, even in Germany. Here, tacit knowledge (Collins 2001) about the supposed, or even imagined, abilities of the polygraph works in reciprocal ways, even though its depiction stands in contrast to the actual knowledge in the field. We argue that the cinematic images of the polygraph test and their implied fictional 148 knowledge support the credibility of both scientific and applied lie detection by grounding them in an analog materiality.