I think that Marlow is shown to be different aspects of the same form in Coppola's film and Conrad's book. In the book, Marlow is sent to fetch Kurtz, while the film is quite explicit about Willard needing to "terminate with extreme prejudice." Another difference would be that Willard kills Kurtz at the end of Coppola's film and this is not seen in the book. Another difference would be the post- Kurtz treatment in both. In the film, we really don't see life after Kurtz. Willard leaves the compound, but we don't get the vision of post- Kurtz Marlow in the book, who seeks out Kurtz's fiancee to protect her and say that he died with her name on his lips. Finally, Marlow is a detached Buddhist like figure in the book, while in the film, we see him as almost as savage as Kurtz. While we may feel many things about and for Willard, seeing him as a detached sage is not one of those experiences.