There are all sorts of things which may be done to correct matters if so required. Don't let anyone leave during a station. It's disruptive for the other candidates. Plus, the candidate involved loses possible marks. The Old Wide- Question Ploy.
Many candidates now adopt a ploy of asking a very wide or a multiple-question.
The answer is to tell the candidate which one of their questions that you are answering. The role actor should be prepared to use the services of the assessor during the one and a half minute break between the stations if, for instance, they find that they are getting into role. The assessor should be watching for this and for inconsistency etc. There must be no role-playing, just acting. Each candidate must have the same station.