Corrected entry: At one point, it is revealed that the Janitor's name in the show is Neil Flynn (the actor's name), and J.D. sees him in a bit part he played in the Harrison Ford movie "The Fugitive." So, in essence, while everyone else are fictional characters, Neil Flynn is playing himself - as a sadistic janitor.
Rooster of Doom
27th Jan 2004
Correction: The Janitor's name is never actually revealed; Neil Flynn's real-life past was used to give his character some depth, but the latter remains known to all and sundry only as 'Janitor'.