Lying is assumed to increase cognitive load, and it has been shown to slow response times to simple questions. We employed a dual-task methodology, the detection response task (DRT), to assess cognitive load in telling narrative lies in a live, open-question interview. The DRT requires participants to press a button in response to a tactile stimulus every 3-5 s while performing a primary task, in this case, recounting either truthful or deceitful narratives. We found weak support for slowing in the time to initiate a narrative response when lying. In contrast, we found strong support for an increase in cognitive load when producing a narrative lie, as measured by both slowed DRT responses and increased response omissions, although this effect decreased with time on task. We advocate dual-task methodologies such as the DRT for increasing understanding of the assumptions made by theories of deception and for refining lie detection techniques.