The model paraded in front of the celebrities with panache; it was impossible to tell that this was her first assignment.

Clue: The second part of the sentence tells us that it was her first assignment, but that it was impossible to tell that. Hence, since she was not behaving as a novice might, she must be behaving with panache (in a confident and stylish manner).
(opprobrium = shame; trepidation = fear)