The basic summary of "A Message to Garcia" is to get it done by any means necessary. I could give the synopsis and go into detail recounting the story, but I will not because I know you have read it. The reason that you would want any candidate to give an account of the story's importance is because it's about one person, Rowan, with limited resources making things happen. No excuses. No explanations. Just getting things done. This is important to anyone looking to become an executive assistant, especially given the outline of the opportunity your team has presented. 

In actuality, that trait is important for anyone looking to be successful in whatever they undertake. Rowan is decisive, measured, blunt and determined; all skills necessary to ensure success. Perhaps the most important point about the entire piece is that is not only relied on his judgement but the judgment of others...when the time was right. He did not burden others with unnecessary questions and only sought input after he exhausted his options and required assistance. Undoubtedly, this story should serve as a blueprint for the attitude you expect of the person who assumes this role.