I found it like an onion, peeling layers into the pressures of acting; movie actors trying to be accepted in to theatre when roles dry up for them, so it becomes about 'real' talent. I also liked the lure of the easy pay day against the slog and risking wealth and reputation on the stage a la the voice in his head. He hates 'Birdman' like Adam West hated 'Batman' and I can see the correlation of the characters and those defining roles all they were ever known for.
It's brilliant the way he must close to, or actually, bipolar as he doesn't get his hands dirty when he flips cos he blacks out.
I loved this film on so many levels.