It would be ideal to be able to assess the raw talent of all candidates and get their honest answers. However as pretty much most candidates who want a place on this scheme will practice and rehearse, then it would be unwise for the naturally talented to not do any prep as the rehearsed candidates will probably outperform them in the interview setting.
Perhaps the more effective solution would be to ask more scenario type questions that aren't easily rehearsed without knowing the question, or from a previous career, they gave you a 20page document 48hours in advance, which was then assessed at the interview to test whether you had the learning capacity required to complete the course. This felt brutal at the time, but then filtered out those who were not able to retain large volumes of information in the timescale given.
I don't think there is an easy answer for recruiters, but for candidates, preparation is essential to succeed, regardless of natural ability