In the Prologue of Shakespeare's Romeo and Juliet, Shakespeare characterizes the piece as a tragedy primarily through the use of language. As with any piece of writing, the very beginning sets the mood and gives the audience (or reader) some sense of what kind of tale is to follow. In this case, the language infers that the play is a tragedy and the Prologue tells as much. The Prologue is only fourteen lines long, so that each word must be carefully used to convey the author's intent.
(The Prologue is written in poetic form:
The Prologue to Romeo and Juliet is also a sonnet...
It has fourteen lines, each is written in iambic pentameter, there are three quatrains and a rhyming couplet, and the rhyme scheme—or "pattern of rhyme"—follows that of a Shakespearean—or Elizabethan—sonnet.)
Because of its brevity, the selection of words is important to the structure of the "sonnet," which in this case is the Prologue. However, it is not the structure that provides insight to the audience. (The structure makes the Prologue sound "poetic.") The concise use of language, that happens to follow the patterns of the sonnet, "speaks" to the audience.
Shakespeare makes use of strong and specific language to characterize the play as a tragedy. There is an extensive list of words and phrases that he employs:
ancient grudge, new mutiny, blood, civil hands unclean, fatal loins, foes, star-crossed, take their life, piteous, their death bury, strife, fearful passage, death-marked, rage, and children's end
With the phrase “from ancient grudge, break into new mutiny,” we can infer that this will be a tragedy because the play would not simply be about an age-old feud—because it would not provide any plot development; that the story moves toward a tragic end is indicated by “new mutiny.”
The language introduces the sense of a deadly conflict, which would infer the presence of tragic elements, and it also alludes to “human suffering.”
To be truly effective, the very beginning of the "story" must make the best possible use of the writing therein to set the mood and introduce the audience to the kind of tale that will unfold before them. Shakespeare, a masterful writer (perhaps the best in the history of the English language) uses the careful selection of words, and “word economy,” to concisely “set the stage” for his play.