As a matter of fact, have you never noticed that most conversations are simply monologues delivered in the presence of a witness?
Margaret Millar (1915-1994) American-Canadian mystery and suspense writer
The Weak-Eyed Bat (1942)
(Source)
Often misattributed to Mark Twain, usually as "Most conversations are simply monologues delivered in the presence of witnesses."
More information on this quote's origins: Most Conversations Are Simply Monologues Delivered in the Presence of a Witness – Quote Investigator®