How does epic poetry differ from prose writing or drama?

Epic poetry, more so than prose or drama, is rooted in oral history. It draws upon the subject matter common to those stories which were recited, in public, within pre-literate societies. It follows, then, that epic poetry relies on rhyme and meter, and has a song- or trance-like quality. When we read an epic poem, like The Odyssey, we can almost hear the voice of an ancient story-teller.

Whereas prose is written in "spoken" or "ordinary" language, without metrical structure, most epic poems make use of meter. The Odyssey, for example, is written in dactylic hexameter. Epic poems also focus on heroic and otherworldly ideas, events, and people; the world of The Odyssey is populated by gods, monsters, and super-human figures like Achilles and Odysseus. Prose, on the other hand, is more variegated in its subject matter. One can write prose about almost anything, be it everyday life, incredible battles, or life on other planets. 

Epic poems, unlike prose and drama, rely heavily on epithets (such as Homer's "rosy-fingered dawn") and repetition. They usually begin in media res, or in the middle of the action, and focus on a hero's journey. Heroes may encapsulate the values of an ancient civilization, while simultaneously challenging or building upon said values. There may be an invocation, in which the narrator invokes a Muse or other divine figure, and a praepositio, in which the main theme of the epic is clearly stated (usually at the very beginning of the work). Genealogies and national histories may be given (known as enumeratio).

While drama and epic poetry may share common origins - that is, oral history, performance, and story-telling - they diverge in critical ways. Drama, like prose, can take on any subject matter, and thus is more variegated than epic poetry. Drama is currently performed, whereas epic poetry is not; epic poetry also assumes a single story-teller or narrator, while a dramatic performance may involve a number of speaking characters. Further, epic poems tend to be cyclical, with a main hero vacillating between crushing defeat and massive triumph. Flashbacks are also common, and the narrator may interrupt a scene of action with a genealogical history of the warriors involved. In contrast, dramatic action tends to be linear, with a climax and final resolution. Imagine how confusing - and annoying - it would be to watch a play in which the main character was caught in an endless cycle of defeat and triumph, with no resolution in sight!

Last, it is uncommon for a work of drama to begin in media res. Again, try to imagine how different, and how bizarre, Romeo and Juliet would be if it opened in Juliet's tomb, with Juliet about to stab herself to death. If Juliet were to follow the conventions of epic poetry, she would have to stop, put down the blade, give a praeposito, invoke a Muse, and explain her current predicament through a series of flashbacks. She may also have to recount her family tree, and one or more epic battles. We see, indeed, that drama and epic poetry are separate literary entities. Ultimately, it is the shadow of oral history which, more than anything else, separates epic poetry from both drama and prose.