This is a great question! I think the film is an incredible reflection on Conrad's original novel and has so many comparisons that you can draw, in addition to so many differences.
For me, the key four differences between the Marlow in the book and the Marlow in the film are as follows:
1) In the film Marlow is an assassin whose job it is to kill Kurtz. In the book he is paid as a pilot to take the steam boat down the river into the heart of darkness, to Kurtz's station.
2) Marlow in the book is a very reputable hard-working individual who wants to do a good job and in a sense keeps himself sane by focussing on his work and his dedication to his work. In the film, Marlow is shown as a druggie who needs work to stop himself going into a destructive spiral of drugs and drink.
3) Marlow in the film is an experienced soldier with lots of field service in a variety of locations. In the book, Marlow is a stranger to the colonial Africa and the sights that he sees.
4) Lastly, in the book we get to see Marlow before he goes to Africa and afterwards, whereas in the film we only see Marlow in location in Vietnam.
Hope these differences help! Of course, there are lots of similarities to look at as well...