Actor Allusion: As Warren, Jack Burns would occasionally use his stand up trademark of making a statement to another character then saying "huh?" "huh?" "huh?" until the other character shouts an agreement with him to get him to shut up.
The Danza: Andy Griffith as Andy Taylor and Elinor Donahue as Ellie Walker
Fred Ziffel is a hobo Barney chases away from loitering on the railroad tracks.
From the same universe, Mr. Drucker is the town drunk in Mt. Pilot, whom Otis befriends when he "runs away". In fact, actor Frank Cady was originally tested for the part of Otis himself, but was determined to not be right for the role.
Griffith is a native Tarheel. Mayberry is a pastiche of Mount Airy, where he grew up. Nearby to Mount Airy is Pilot Mountain, renamed to Mount Pilot for the show.
Being on the show so endeared the state to Frances Bavier that she moved there (specifically to Siler City) later in life.
Howard McNear (Floyd The Barber) had a severe stroke during the run, and upon returning could not walk or stand on his own. He thereafter was always shown sitting or leaning against an object, and would "walk" across the room off camera. When worsening health required him to leave permanently, Floyd retired and the barbershop became Emmett's Fix-It Shop.
At least two different episodes has Barney desperately wanting the big solo in choir - in one such episode, he loses to Gomer, whose singing voice is much more suitable for the solo; in the other, he's over confident that he's got the solo while the rest of the choir try to find a way to get rid of him without hurting his feelings.
In a more literal sense, as mentioned about, scripts were already written with Barney in them for the beginning of Season Six, but with Don Knotts not returning, Barney was simply replaced with Warren in the scripts.
What Could Have Been: Originally, Andy was supposed to be the comedic lead with Knotts as his straight man (similar to their roles in No Time For Sergeants, but they quickly discovered it was funnier the other way around: "By the second episode, I knew that Don should be funny, and I should play straight."