The aim of the paper is methodological. It deals with data collection methods. It focuses on two methods: observation and verbalizations in self-confrontation. In ergonomic psychology, it is often suggested they should be used jointly. In the 1990s it was suggested that their results should be compared to reduce biases from verbalizations in self-confrontation. In the 2000s, researchers interested in studying activities in the service field also suggested using them jointly; this time the verbalizations in self-confrontation were considered as a means of correcting errors coming from behavior analysis.The purpose of this paper is to present their use in an investigation process through a case study. The analysis process used for each method is explained and illustrated. The analysis methods and models borrow from interactionist linguistics and ergonomic psychology. A case study (excerpt of a home sale) was chosen because it refers to a complex work activity: a dynamic activity accomplished through speech acts.The results show empirically how these methods used jointly enable us (i) to enrich knowledge capitalization and (ii) correct some results. Their strengths and limitations are specified. For instance, self-confrontation used alone provides access to the purposes, actions and knowledge verbalized. The knowledge collected in self-confrontation is partial: the professional treats relevant aspects from his/her point of view. Behavior analysis used alone is not always sufficient to identify the professional’s purposes and related functional actions. It does not allow access to sales skills and the trade’s specialized terminology, but it allows the discovery of skills not accessible in self-confrontation.