In Romeo and Juliet each character has a specific role. Mercutio's role is the most important. He is the kinsman to the prince and Romeo's closest friend and confidante, and in this role he can be very inluential to Romeo and can influence the decisions made and the directions the play takes. He uses his humour and wit to lead the other characters to the climax. He is in a difficult position, as the friend of Romeo and the princes kinsman, but all these factors contribute to the memorable character he is.
Mercutio is the driving force behind the play, as he fortells or is the cause of most of the major events.
Before the Capulets party Benvolio advises they enter quietly, dance and leave, but Mercutio, the only person in this scene actually invited to the play, disagrees;
'duns the mouse, the constables own word,
if thou art dun, we'll draw thee from the mire'
Mercutio is saying that only a mouse should be silent and small.
In this instance Mercutio is moving the play along by teasing Benvolio and Romeo into staying at the party longer, which turned out to be fatal.
The party is a major event in the play because Romeo meets Juliet there, and consequently decides his love for Rosaline was nothing but a crush. The next major event in the play is Mercutio's own death, this is often believed to be the climax in the play because from there on everything changes for the worse.
In Mercutio's last scene Romeo intervenes as Mercutio is fighting Tybalt and Mercutio is wounded under Romeo's arm, Mercutio then makes Romeo question why he intervened and then to feel responsible for Mercutio's death, he seeks revenge on Tyalt and is banished, which provides the complication for the story.