Writing an expository essay which tries to emulate the voice of the studied text's author sounds tricky. To me, it seems like your teacher is rather fond of the creative pieces. Perhaps give a creative piece a try? I don't think expository would really work if you are writing in style of The Quiet American. Even if you did incorporate the language of the text into your essay, I doubt a marker in the external exam would even pick up on something so small in the minuscule amount of allocated time they have per paper. All this being said, I did expository in the exam last year (and every context SAC) and I never wrote in the style of the text - only explored similar themes and ideas in my own original piece.
Hope I haven't confused you by going against what your teacher said. Maybe have a chat with him/her?