Obviously this is a character study and you will need to look in detail at those scenes in which Mercutio is present and at his speeches.
Build up a picture of the man in the play, Romeo’s friend and adviser, defender of his name and ultimately sacrifice to Romeo’s honour.
This question gives great scope for you to work in the sources – Brooke’s poem and the expansion Shakespeare gave to the characters of Tybalt and Mercutio; decide why too.
Could also include the place of omens and destiny because of the Queen Mab speech and how he was just as doomed as R and J.
In the film consider the way Luhrmann casts him: black, drag Queen, volatile, emotional… and what other cinematic effects are brought to bear: the storm gathering as he curses them…
Consider how indispensable Mercutio is to Shakespeare’s retelling, i.e. what would be the effect on the play if he wasn’t in it? And you immediately see how his role is relied on to provide the impetus for the final tragedy and the events which follow; his death precipitates Romeo into the killing of Tybalt, this leads to Romeo’s banishment and the inevitable confusion over the message from Friar Laurence.