George Bernard Shaw is very much the best known playwright in this regard.
That is because Shaw's (nominally non fictional) prefaces and stage directions describing the fictional characters' lives and the play's fictional locales were all very much longer than the scripts of his fictional plays.
While Shaw encouraged stage directors and actors to freely ignore these non-fictional efforts in order to create their own original staged performances, he hoped lay readers of his plays would read them and the script as all as being of one piece.
What should interest me, if I were a young academic, is the mind-stretching fact that even the bare script of a fictional play also includes non fiction fact.
For while the actor may be playing the role of a fictional military man, he unerringly 'enter stage left' as the script claims at the beginning of Scene Two in every performance and that is a fact - ie non fiction.
Stage directions in fictional dramas about real life characters and events
But let us instead imagine that Shaw had been a playwright oriented to creating fictional scripts based closely on the life and times of real people and real events.
Then, if he was consistent, one should expect his stage directions to become fully non-fictional as they reveal for us the real life and the real settings behind his fictional prose play.
(Or music drama - because Shaw had an equally illustrious career as a music critic as he had as a playwright and in other circumstances, one could easily image his plays as modern operas or musicals. Er, My Fair Lady anyone ?)
If read then as one piece, the entire script/libretto and stage directions would very closely resemble Melville's Moby Dick in its free mixing of fact and fiction reflecting on the reader's understanding of each.
That is, in fact, my aim in my libretto...