We present results of a detailed analysis of ten screen capture videos documenting how participants in a laboratory user study used two different interfaces to complete data exploration tasks. Findings presented in this paper include methodological observations as well as those concerning the design of both interfaces. On the methodology side, we demonstrate that observation and analysis of videos that capture how people used a system can uncover aspects of interface learnability, different approaches to problem solving and efficiencies associated with good design. In the context of the two compared interfaces, we document the details of how participants used each interface: the affordances they used and patterns of interface exploration they followed. Combined with the data on task completion time and accuracy, these analyses provide empirical evidence to our assumptions regarding what makes a particular interface efficient. These analyses can greatly inform re-design efforts after pilot evaluations, complementing time and accuracy data. They can also help guide user training and automated user support.