In what ways does the presentation of Shylock differ between the original text and the film, and why are there differences?Comparing the novel...

I assume you mean the recent-ish Al Pacino version of Merchant of Venice. Well, linked to your question, one of the crucial decisions any director of this play needs to make is to decide how to present Shylock. An essay I get my students to do which looks at this goes like this: "Shylock: Villain or Victim?" Discuss. Basically in the text there is evidence to support both views so you need to decide how you wish Shylock to appear to the audience.

One of the crucial additions that the film of the play makes is that it starts the film with information talking about how Jews were persecuted in Venice at the time of the play, and shows this persecution, with Antonio spitting on Shylock. This supports the view that Shylock is a victim much more, having faced horrendous persecution and championing his people. The scene at the end of the film is also very poignant, where Jessica, although she is with her beloved Lorenzo, still obviously feels pity for her father, who has been forced to convert. The last scene where we see Shylock he is alone and an unbelievably tragic figure. The film therefore strongly supports the view that Shylock is a victim, which, inspite of his bloodthirsty nature and his singleminded pursuit of his "merry bond", is the impression that we take away with us.