Critics consider Mercutio to be one of Shakespeare's most interesting characters. He is both a round and static character, which makes him extremely unique in literature. Characters are usually flat and static or round and dynamic. A round character has several different character traits as opposed to a flat character who may only display one or two traits. A dynamic character changes as a result of the events in the story while a static character does not change much during the course of events.
Mercutio is certainly round as he displays different characteristics. He is flamboyant, maddening, belligerent, stubborn and loyal. He is flamboyant in his use of language. His Queen Mab speech in Act III, Scene 4 is imaginative and intense. He conjures fascinating images such as a "fairies midwife...no bigger than an agate stone." He references both violence ("cutting foreign throats") and sexuality ("maids lie on their backs").
He is also quite maddening. He won't listen to Benvolio's pleas to get off the street in Act III, Scene 1, despite the decree of the Prince earlier in the play. Instead, he accuses Benvolio of being unreasonable and always looking for a fight. He says,
Thou—why, thou wilt quarrel with a man that
hath a hair more or a hair less in his beard than
thou hast. Thou wilt quarrel with a man for cracking
nuts, having no other reason but because thou
hast hazel eyes.
He is belligerent in his attitude toward Tybalt in the same scene. He is ready to fight from the outset. He refers to his sword immediately after Tybalt accuses him of "consorting" with Romeo
Here’s my fiddlestick; here’s
that shall make you dance. Zounds, consort!
Mercutio also shows his loyalty to Romeo and the Montague cause when he stands up for Romeo. He cannot allow his friend to be called a villain, so he steps in to fight Tybalt. Finally, he is also stubborn in his death scene. He never changes and attempts to be witty to the very end, punning on the word "grave" while expounding on his belligerence in cursing the feuding families:
No, ’tis not so deep as a well, nor so wide as
a church door, but ’tis enough. ’Twill serve. Ask for
me tomorrow, and you shall find me a grave man. I
am peppered, I warrant, for this world. A plague o’
both your houses!
For this very reason, Mercutio is a static character. He is flamboyant and belligerent to the very end. He never gets beyond his set characteristics. To be a dynamic character he would have to show some emotional growth which he never does. He is fighting and joking to the end of his life.