This is a tale I have written about before.
I worked on the original Ain't Misbehavin' and one of the stage managers gave a quick insight as to why the show works so well.
There is a subliminal "book". Maltby decided to make it the life story of Fats Waller and each song is placed in the show where it is placed for a reason. Ain't Nobody's Business If I Do was given additional lyrics to tell Fats' philosophy for instance. The first act has more youth and exuberence while in Act Two, the show goes to dark places at times (The Viper) and also into high society, places where Fats went.
Its all there and Maltby has never spoken about it. I don't know where anyone else is who would know the details. It is this 'book' that separates this show from most of the other revues out there and makes it a superior one.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher