Sort of depressing and also sort of uplifting: Raymond Carver's quote used to open the movie can also be found on his tombstone. (Source)

Riggan's third Birdman movie came out in 1992, the same year that Michael Keaton's second and last Batman movie was released. Just like Riggan refuses doing more Birdman movies, Keaton chose not to continue his Batman career. (Source)

The electric light on a pole, seen during Mike's first appearance and during the short cuts after Riggan shoots himself, is known as the ghost light. It's used to provide lighting to help late goers and early comers avoid falling into the orchestral pit (and to scare away ghosts, of course). (Source)

When we first meet Mike, he immediately begins suggesting edits to Riggan's script, being the prestigious, know it all actor that he is. Before doing that scene, Edward Norton was picking apart the Birdman script with Iñárritu, who pointed out the irony to Norton. (Source)