We can certainly learn a great deal about how Shakespeare portrays his characters in Romeo and Juliet through looking at his use of language. While we won't have space to discuss many characters, below is an analysis of Benvolio's language to help get you started.
Benvolio is one of the only sensible and rational characters in the play. He is also a peacemaker while other characters only want to stir up the violence. We can see just exactly how Shakespeare portrays him as a sensible, rational, strong, assertive peacemaker through the language of the very first lines that Benvolio speaks, "Part, fools! / Put up your swords. You know not what you do" (I.i.59-60).
First, these lines show Benvolio's sense of moral superiority. Shakespeare especially shows Benvolio's superior sense of morality with his diction choices in Benvolio's opening words, "Part, fools!" Calling the servants "fools" tells us that he perfectly sees the servants are behaving foolishly, irrationally, and immorally by engaging in a fight for no reason. The fact that Benvolio can rightly observe their behavior as foolish shows us that he has a moral understanding that is superior to the servants' and even superior to every other character that behaves in the same way the servants do. In addition, the shortness of the line and use of the exclamation point helps portray Benvolio as strong and assertive.
Another important device Shakespeare used for Benvolio's language to portray him as rational, sensible, and morally superior is called a hyperbaton. Hyperbaton refers to an author's choice to refrain from using normal, grammatical word order. Dr. Wheeler gives us the example, "One ad does not a survey make" ("Schemes"). Hyperbaton can be seen in Benvolio's line, "You know not what you do" (60). In normal word order and grammar, this would be written as, "You do not know what you are doing." However, Shakespeare's shortened version without normal word order allows us to see the emphasis on the words "know," "not," and "do." Emphasizing these words shows us again how Benvolio is morally judging the servant's actions, telling them how little they know, and emphasizing the word "do" to emphasize the wrongness of their actions.