Assess the function of Mercutio in the play. Why is he so important to the play and to the development of its themes?I'm trying to think about...

A great question. Ostensibly, Mercutio is a side character in a tragicomic play about two lovers who are the protagonists, so at first glance, he may seem only like a sideliner and not that important. But in literature, great literature especially, every character counts. So if you want to move beyond a "C" in your literary analysis, treat everything in works by greats such as Shakespeare as seriously symbolic.

So, let's examine Mercutio carefully in terms of function and personality. You're already doing a good job of characterizing his personality. "Light-hearted" is a characteristic you can easily prove. Search acts one through three for instances of his joking around, particularly his bawdy ones, and you'll have plenty of evidence of his ability to get a crowd laughing.

You say he is a "good friend." This is an evaluative judgment (any time you put "good" or a criterion-based adjective, you're evaluating), so you need to back this up. What makes a good friend? Make a list of good qualities in a friend, and then see if Mercutio measures up. Is a good friend loyal? Thoughtful? Compassionate? (Rate Mercutio in each of these areas in how he treats Romeo.). I will challenge you to think about the kind of friend he is and come up with a more specific adjective than "good," because "good" is a bit too general for a complicated guy such as Mercutio. Some would argue that his kind of "good" does Romeo no good! But there are many ways to argue this.

You say "a man of many words." True, he is that, but what kind of words? Do you mean talkative only, or a man who says a lot of a certain kind of statement? Take a moment to examine the Queen Mab speech (act one, scene four), and really study the images that Mercutio raises. Are they pleasant, interesting, exciting, gruesome images -- what kind? I would use that speech to characterize the kind of talking Mercutio does.

Now on to function. This is using the same strategies we've just been using, but asking you to think more globally about Mercutio's role in the play. Find two to three key decisions that he makes (hint: act one, scene four, he gives advice to Romeo in the line, "Nay, gentle Romeo...," and that qualifies as a decision. Then, in act three, scene one, he makes a life-changing decision.

Now ask yourself: how do Mercutio's key decisions affect the course of the entire play?

Finally, think of the concept of friendship as a concept that weaves through the whole play. Benvolio, Mercutio, the Nurse, and the Friar can all be counted as friends to the main characters, Romeo and Juliet. In terms of being "good friends," who do you find to be the best, and why?

Therefore, when you answer a question such as the one you posed, you can answer it by characterizing Mercutio (listing his most important qualities in terms of quality or goodness), by listing his key decisions that affect the plot, and by evaluating him in light of his role as friend (when friendship is a concept that is dealt with again and again).

Note the distinction between concept and theme: a concept is an idea, a topic, such as "friendship," "love," or "loyalty." To write a thesis statement about a theme, you need to make a claim, as in, "Mercutio is a ____ type of friend and in this play compares to other friends in the following way..." You want to make an overall statement about how an idea -- a theme -- gets played out via characters' choices and roles.

Good luck!