The Great Pretender
Mike, like the open book he is, pretty much tells us the key to his identity:
MIKE: I don't pretend out there, I told you. I pretend just about everywhere else, but not out there.
This is Mike in a nutshell: a man who cares more about authenticity than he cares about anything else, but whose own authenticity ends where the stage ends. "Out there" (or "onstage") Mike is magic, and not in a Channing Tatum sort of way. Even brutal theater critics like Tabitha praise his every performance. And just look at the reaction Riggan and Jake have when they hear that the legendary Mike Shiner is about to grace their play with his performance.
Or can we even call it a performance? On stage Mike isn't acting, he's being.
The problem is he expects this kind of commitment—and weird approach to onstage vs. offstage reality—from everyone else. When the audience is videotaping the performance on their phones, he berates them for living their life through their screens and yells at them to "have a real experience."
He then proceeds to figuratively and literally rip apart the set, proclaiming it to be fake…like he's the prophet of truth and not just an actor who gets so into character that he gets drunk when the stage directions demand it.
Truth Is Stranger Than Drama
Of course, the problem is that Mike can't replicate this kind of authenticity when he's not acting. He ends up being more of an actor in his day-to-day life than when he's performing. This is creepy at best. As Lesley puts it:
LESLEY: Maybe up here you're Mr. Truth, Mike, but in the real world, where it counts, you're a fraud. How's that for truth, you dick!
And this isn't just Lesley being emotional; we see this fraudulence over and over again in Mike's actions. When talking to Sam he tells her she's "kinda a great mess, it's like a candle burning on both ends, but it's beautiful." He also talks about how he wants to:
MIKE: […] pull the eyes out of your head and stick them in my skull, and then look out at this street and see it the way I saw it when I was your age.
This is some straight-out-of-a-play, overly analogous, hyperbolic, symbolic dialogue. (Sam even calls him out for it.)
And when The Times comes to give him an interview, he makes up all sorts of things about having had a drunk as a father, and about how hard it is to work with a dunce like Riggan, etc. Mike doesn't think twice about lying in this way…which is exactly the opposite of what he demands from his audience and fellow actors when he's on stage.
We never really get much closure on Mike's development, but when he's talking to Sam near the film's end, we sense he's beginning to have a change of heart. Sam's impressed by Mike's indifferent demeanor:
SAM: You really don't give a s*** if people like you or not?
MIKE: Not really.
SAM: That's cool.
MIKE: Is it? I don't know.
This shred of self-doubt is super-important—Mike's beginning to rethink his whole "acting offstage and being realistic onstage" shtick.
And there's another moment that underlines the fact that hairline fractures have appeared in Mike's stony, performative exterior. When Sam's shocked that her father gave Mike a black eye he shrugs it off saying,
MIKE: No, no, I think maybe I deserved it.
Owning up to his flaws means Mike's come a long way from being obsessed with his own prestige as a theater actor, and starting to realize that "dealing with complex human emotion" can happen offstage as well as on.
Good job, Mike. You get a gold star.