Communication assessment in real-life consultations is a complex task. Generic assessment instruments help but may also have disadvantages. The generic nature of the skills being assessed does not provide indications for context-specific behaviour required in practice situations; context influences are mostly taken into account implicitly. Our research questions are: 1. What factors do trained raters observe when rating workplace communication? 2. How do they take context factors into account when rating communication performance with a generic rating instrument? Nineteen general practitioners (GPs), trained in communication assessment with a generic rating instrument (the MAAS-Global), participated in a think-aloud protocol reflecting concurrent thought processes while assessing videotaped real-life consultations. They were subsequently interviewed to answer questions explicitly asking them to comment on the influence of predefined contextual factors on the assessment process. Results from both data sources were analysed. We used a grounded theory approach to untangle the influence of context factors on GP communication and on communication assessment. Both from the think-aloud procedure and from the interviews we identified various context factors influencing communication, which were categorised into doctor-related (17), patient-related (13), consultation-related (18), and education-related factors (18). Participants had different views and practices on how to incorporate context factors into the GP(-trainee) communication assessment. Raters acknowledge that context factors may affect communication in GP consultations, but struggle with how to take contextual influences into account when assessing communication performance in an educational context. To assess practice situations, raters need extra guidance on how to handle specific contextual factors.