The fact that Mueller could NOT have taken the position of FBI director, convinces me that it was not an interview, it was telling Mueller his roll in the movie.
Mueller (Rosenstein also) was so comped that he has no choice but to act as a player.
http://thehill.com/homenews/administration/337182-trump-considered-mueller-for-fbi-director-before-he-was-named-special
https://www.pbs.org/newshour/nation/4-candidates-trump-interviewed-fbi-director
https://en.wikipedia.org/wiki/Director_of_the_Federal_Bureau_of_Investigation