They're not doing actual prep for specific opponents until the tournament begins.
Exceptions are the Candidates and the World Championship.
@#1
http://edition.cnn.com/2015/08/21/sport/nakamura-chess-grandmaster/index.html
a 2 year old article, 40 questions
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Does anyone know of articles or interviews in which top GMs discuss how they prepare for tournaments and/or specific players?
Also, any articles/interviews with GMs outside of the elite who make their living from playing chess describing their lives?