This paper introduces CM-DIR (Conceptual Model -Dynamicity and Interaction in Recommender Systems), a method that we have originally devised to help with the description of dynamic user behavior in Recommender Systems (RS). The identification of user behavior change is paramount to optimize RSs to face new contexts of use and adapt their user interface accordingly. Capturing dynamic user behavior is, however, not an easy task and most RS systems do not take this information into account during the requirements engineering process. The proposed method leverages user and Behaviour-Driven Development (BDD) stories and extends them to describe dynamicity. The method has been evaluated with a panel of experts through semi-structured interviews and the results suggest that the CM-DIR is necessary and comprehensible and that the proposed extension allows flexibility and adaptability while ensuring informative specifications.