The effectiveness of faculty development (FD) activities for educators in UK medical schools remains underexplored. This study used a realist approach to evaluate FD and to test the hypothesis that motivation, engagement and perception are key mechanisms of effective FD activities. The authors observed and interviewed 33 course participants at one UK medical school in 2012. An observed engagement scale scored participants' engagement while interviews explored motivation for attendance, engagement during the course and perception of relevance/usefulness. Six months later, using the realist framework, 12 interviews explored impact on learning outcomes/behavioural changes, the mechanisms that led to the changes and the context that facilitated those mechanisms. The authors derived bi-axial constructs for motivation, engagement and perception from two data-sources. The predominant motivation was individualistic rather than altruistic with no difference between external and internal motives. Realist evaluation showed engagement to be the key mechanism influencing learning; the contextual factor was participatory learning during the course. Six months later, engagement remained the key mechanism influencing learning/behavioural changes; the context was reflective practice. The main outcome reported was increased confidence in teaching and empowerment to utilise previously unrecognised teaching opportunities. Individual motivation drives FD participation; however engagement is the key causal mechanism underpinning learning as it induces deeper learning with different facilitating contexts at various time points. The metrics of motivation, engagement and perception, combined with the realist framework offers FD developers the potential to understand 'what works for whom, in what context and why'.