This paper describes the processes used to prepare the role players who simulate the 'patients' in the MRCGP Clinical Skills Assessment (CSA) examination, one of the licensing qualifications for GPs completing their training. Because it is such a high-stakes exam, the CSA must be seen to be fair and trustworthy, and such attributes could be compromised by inadequate role player performance. On a CSA examination day, three role players simulate the same 'patient' simultaneously on three different exam floors, each playing the same case up to 26 times on the day, and these 'patients' must present in a consistent and reliable manner to ensure that all candidates face the same test situation. The paper describes the initial selection and training of the actors, how they are prepared on the day of the exam itself, and the quality assurance processes designed to ensure that the exam has excellent role players performing as the simulated 'patients'.