Considering the exposition of a plot involves thinking through the beginning of the plot and how the characters and the conflict are established. There are certainly ample differences between these two texts. In the film, we are presented with a Marlow who shows he is struggling with not being on duty and is clearly a troubled individual. He drinks, takes drugs and self-harms himself and it is clear that he is a troubled man with a troubled past. However, in addition to this internal conflict, he is given a new mission - to voyage up the river and find and eliminate Colonel Kurtz - a clear external conflict that he must resolve. What is interesting about the meeting is the mystery surrounding Kurtz and his apparent decline into "madness" which immediately creates a link with the Marlow who we have just seen naked punching a window.
In the novel, Marlow is not presented at all in this way. He certainly has a thirst for adventure and the mystery of Africa and travel is something that draws him in, but at the same time, he shows himself to be a reasonable, hard-working individual who needs work and is able to use the contacts that he has (his Aunt being a prime candidate) to get the position that he desires. His mission is only to pilot the steam boat up the river to the "heart of darkness," not to kill Kurtz.