As its name states, it is a monologue, hence it is a speech uttered by a single person. In its fullest form, the dramatic monologue has three salient features Firstly, a single person who is patently not the poet, utters the speech that makes up the whole of a poem, in a specific situation at a critical moment. The dramatic monologue must occur at a critical moment, as it is this feature that distinguishes the dramatic monologue from a monologue. It also gives a dramatic effect, as it gives the reader the feeling that the something has happened before which has led to the utterance of the dramatic monologue. For instance, in the poem, The Forsaken Merman by Matthew Arnold, the Mermans human wife has abandoned him and their children in order to go pray in a church. The Merman utters the poem at the time when he realizes that that his wife will never come back.
Secondly, this person addresses and interacts with one or more other people, but we know of the auditors presence only from clues in the discourse of the single speaker. The auditor never speaks, but we know of what he or she says and does when the speaker tells us. For instance, in the poem The Love Song of J. Alfred Prufrock by T.S. Eliot, the implied auditor is Prufrocks lover. We know of her presence when Prufrock addresses her, for example Let us go then, you and I. This first line of the poem tells us then that the poem is addressed to a specific person. Another instance is Oh, do not ask, What is it? This line tells us that the auditor, that is the lover does interact with the speaker, Prufrock, but we know about these interactions only through what the speaker says.
Thirdly, the main principle controlling the poets formulation of what the lyric speaker says is to reveal to the reader, in a way that enhances its interest, the speakers temperament and character. This focus on self revelation is what distinguishes the dramatic monologue from its near relation, the dramatic lyric, which is also a monologue at an identifiable moment but it does not reveal any new facts. For example, in the poem Porphyrias lover by Robert Browning, the lover who is the speaker, through the course of the poem reveals his insanity. He reveals that though Porphyria was beautiful and loved him, he was still dissatisfied until the point of time that he realizes that Porphyria worships him. It is at this point of time that he reveals his insanity, when unable to contain his emotions on realizing the extent of Porphyrias devotion, he wound her hair Three times her little throat around | And strangled her. Thus, such revelations are characteristic of a Dramatic Monologue.
A Dramatic Monologue may or may not have all three characteristics. For instance, the poem Dover Beach by Matthew Arnold does not take place at a critical moment. Yet, it is considered a dramatic monologue as it is uttered by a single speaker, and it has an implied auditor, the speakers love, whom he addresses in the last stanza Ah, love, let us be true. Thus the dramatic monologue is flexible as a poetic style.
A Glossary of Literary Terms -M.H. Abrams, 7th EditionPersonal notes from lecturesPoems :The Forsaken Merman Matthew ArnoldThe Love Song of J. Alfred Prufrock T. S. EliotPorphyrias Lover Robert Browning