A monologue consists of a single person speaking to themselves or to an audience. A dramatic monologue is a literary form, usually a poem, in which you have one person, speaking to an audience or "thinking aloud," who is clearly a character and not the poet. Perhaps the greatest creator of dramatic monologues was Robert Browning. His poem "My Last Duchess," (1842) is a monologue in which a medieval Duke tells a visitor (who never speaks) how he had his wife murdered because her manners were too friendly. ("O Sir, she smiled, no doubt/ Whene'er I passed her: but who passed without/ Much the same smile?" The Duke clearly expects his visitor (and us) to believe that this is grounds for murder; Browning makes it equally clear that he is, to say the least, wrong. This is the hallmark of dramatic monologue: the speaker reveals his/her character and motives to the reader, while remaining unaware that he is doing so.
A good contemporary example of a prose dramatic monologue is Alan Bennett's 1980s and 1990s "Talking Heads" plays, which employ much the same techniques as Browning uses.
A good contemporary example of a prose dramatic monologue is Alan Bennett's 1980s and 1990s "Talking Heads" plays, which employ much the same techniques as Browning uses.