Interviewing is known to be one of the most common requirements elicitation techniques. Interviews are driven by a series of questions asked for the purpose of receiving responses that can help understanding the domain and the needs of stakeholders. However, what constitutes a successful choice and ordering of questions continues to be more of an art than a systematic process. We review literature from a broad range of disciplines in which interviewing is widely applied, in order to identify a set of categories for characterizing interview questions. The resulting typology aims at offering an initial coding language for qualitatively analyzing interview content. Such coding language can be further validated for its reliability to enable standardization and community-wide reuse. We offer examples of how such an instrument would help researchers develop and evaluate both descriptive and normative theories of interviewing.