What are the salient features of Robert Browning's dramatic monologues?

The dramatic monologue as developed by the Victorian poet
Robert Browning is a genre in which a single character is speaking to either an explicit
(as in My Last Duchess or the Bishop Orders his Tomb) or undefined (when there is no
immediate audience except the speaker, as in Porphyria's Lover or Soliliquy of a Spanish
Cloister). Unlike the soliliquy of a play, the dramatic monolgue stands alone rather
than being part of a larger drama or narrative.

Typically,
whatever the putative subject the narrator discusses, the most significant thing
revealed is the narrator's character, whether casuistical in the case of Bishop
Blughram, cold and calculating in the case of the Duke of My Last Duchess, or insane, as
in Porphyria's Lover. The overwhelming majority of narrators in Browning's dramatic
monologues are unreliable -- as the poems progress, we increasingly begin to doubt the
truth, sanity, or motive of what they say.