As you may be aware, I rate AN as my #1 favorite. Eventhough the movie depicts Kurtz as an enigmatic larger than life character, the main character to me was Willard. Though I have nothing against Brando's performance, I didn't care much for it. My opinion is that Brando's only contribution is to look really huge(though even that was not intended by Coppola originally - he had a fit person playing that tole in his script) and sort of dangerous. You also have to factor in the fact that he was the bigger star compared to Martin Sheen, which added to the film's selling. Kurtz is a derivative of "Hearts of Darkness", so that role had to be played by some known person. However Coppola intelligently shifted the struggle and focus to Willard.