Acting, Michael Keaton has long maintained, “is an act of pulling your pants down.” And for the film that one-and-all have labeled his “comeback,” “that’s literally true.”

For “Birdman,” the one-time Batman plays Riggan Thomson, a has-been actor, bitter and a little crazy after all the years that have passed since his days of fame and fat paychecks. So Riggan adapts, directs and stars in a stage drama on Broadway. It isn’t going well. He’s hearing voices, the growl of his former superhero character. His co-star (Edward Norton) is a nut. The critics are sharpening their knives for opening night. And then he gets locked outside the backstage door, his robe is ripped off, and he has to make his way through Times Square in nothing but his tighty whities.

“This is CERTAINLY not a vanity project,” Keaton says, laughing. “Riggan has gone to seed. You’ve got to commit to saying, ‘Oh, this guy is going to look rough around the edges. Should I?’”

The Keaton trotting gingerly through “The Crossroads of the World” wasn’t the lean Dark Knight of the ’80s, with something like a full head of hair. His Riggan is paunchy, balding and wears 60-plus years on his face (Keaton just turned 63). But co-writer/director Alejandro Gonzalez Inarritu (“Babel”) says he was the only man for the job.

“It was very brave for Michael to get this naked emotionally, and physically, for a movie,” Inarritu says.

That’s a word critics are hurling at Keaton like never before —“brave.” Because “Birdman” blurs the line between where the one-time Batman actor ends and the Birdman begins, part of a “metadialogue” Inarritu says he wanted to inject in his movie about celebrity and the crazy reputation that can come with it. Edward Norton, an actor famous for being “difficult,” plays a co-star famous for being difficult. And an actor for whom, as The New Yorker’s Anthony Lane put it, “the Bat-Mantle … rested uneasily on (his) shoulders” plays an actor driven a little crazy by a role he played decades before.

“You just do it, have the (testicles) to say ‘This is who I am right now, in this moment,” Keaton says. “Warts and all, you know?”

His post “Batman” years — Keaton played him twice — weren’t necessarily bad. But Keaton was infamously touchy about discussing the role. Pair him up in a romantic comedy (“Speechless”) with Christopher “Superman” Reeve and all tact flew out the window. He’d had enough, and he earned the reputation as somebody you didn’t want to interview. Even now, he doesn’t start stammering until that past is brought up.

“You’re-you’re-you’re talking about the bigger picture, me playing ‘Batman,’ having that history,” he says. Inarritu created “Birdman” with Keaton in mind, but Keaton claims he had no idea that was the case. “Early on, to me it was just a part that maybe 10 or eight or five guys would have been right for — guys who’ve played characters like ‘Batman.’ But I think he settled on me earlier than I realized. I read Alejandro saying it was always for me after making the movie. We never discussed how much of me he saw in this guy.”

Keaton always seemed too cool to sell out, which explained his gripes about decades of Batman questions. But the testiness has faded, perspective has kicked in. Keaton is an actor who most fondly remembers his most comically extreme characters and has landed a great part, one he was born to play, in a critically-acclaimed Oscar contending film. It’s all good. Now.

“I’m human. So your experience makes up who you play. And I’ve experienced despair. I’m not at that desperation part of my career, so I don’t think like that… You’re human, so you’ve had disappointment, emotions like everyone else. I related to that.”