Interviews are the most common and effective means to perform requirements elicitation and support knowledge transfer between a customer and a requirements analyst. Ambiguity in communication is often perceived as a major obstacle for knowledge transfer, which could lead to unclear and incomplete requirements documents. In this paper, we analyse the role of ambiguity in requirements elicitation interviews. To this end, we have performed a set of customer-analyst interviews to observe how ambiguity occurs during requirements elicitation. From this direct experience, we have observed that ambiguity is a multi-dimensional cognitive phenomenon with a dominant pragmatic facet, and we have defined a phenomenological framework to describe the different types of ambiguity in interviews. We have also discovered that, rather than an obstacle, the occurrence of an ambiguity is often a resource for discovering tacit knowledge. Starting from this observation, we have envisioned the further steps needed in the research to exploit these findings.