How is Paul Marshall portrayed differently in the film version of Atonement compared to the novel?
Remarkably, the movie does a pretty good job at staying true to the character of Paul Marshall; there are even moments when his dialogue is exactly the same as in the book, in crucial moments. For example, when offering chocolate to Lola, he tells her in the book AND in the movie to "bite it...you've got to bite it," in a very creepy and, if you are paying attention, telling manner. He is also the king of the chocolate effort in the war, and drones on and on about his chocolate bars, even making everyone a chocolate drink. So, in character, dialogue and personality, they are incredibly similar.
In appearance though, they do seem to differ. In the movie, Paul is fair-headed, whereas in the book, the description we get is that he is a dark-headed man who has a lot of hair; in fact, it describes how "a few dark hairs curled free from his eyebrow" and also how hair grew out of his ears. So in that sense, the physical appearances are different between the movie and the book.
Other than that, the Paul Marshall that is in the book is the same as in the movie; same intentions, same crimes committed, same creepy penchants. I hope that helped; good luck!