It takes no more than a matter of seconds of watching “Birdman, or (The Unexpected Virtue of Ignorance)” to draw the conclusion that Alejandro Gonzalez Iñárritu wrote his dizzying opus solely for veteran “Batman” actor Michael Keaton.

But wishing someone like Keaton would star in his film, Iñárritu said, and having him actually committing to it, are two different things — especially for a man whose career path mirrors the film’s narrative so closely in real life.

“I think Michael was always in my mind, that Michael was always the best for the part, and I don’t think it would be nearly what it is without him,” Iñárritu told me in a recent interview. “I never try to write a script for anybody specifically because it could be very traumatic for me if for some reason the person would not do it. But once I finished the script, I knew that he would be the best choice.”

Expanding into more theaters Friday, “Birdman” stars Keaton as Riggan Thompson, struggling former film star whose life is in the dumps after starring in three blockbuster “Birdman” movies more than two decades before. His apparent salvation lies in a Broadway play, an all-or-nothing comeback piece in which he stars, directs and produces.

Before he launches into the critical preview period, however, Riggan has to confront a nasty nest full of problems, including the personal issues of a grown-up daughter (Emma Stone) he never really knew; a cast of helplessly neurotic actors including an arrogant Broadway star (Ed Norton) who does his best to sabotage the play at every turn; and a vicious theater critic whose all-powerful reviews can either give life to or quickly kill every production that dares to tread the boards on the Great White Way.

Hovering above the potential disaster-in-the-making, though, is Riggan’s Birdman alter-ego, which has become such a part of his life that he appears to take on the character’s mystical powers at times, and is often haunted by the superhero’s gravelly voice. Fending off interviewers who really only care if there will be a fourth “Birdman” film, Riggan knows he will only truly be set free if he can stage a performance to kill off his blue feathered character once and for all.

Iñárritu, whose previous films include the Oscar-nominated “Babel” and the heartbreaking drama “21 Grams,” says the idea, while mirroring the travails of a former superhero star, actually comes from the voices he hears while struggling with his ego.

“There’s a voice that we all have that judges us and punishes us,” Iñárritu said. “The voice that I hear especially during the creative process, that is full of doubts and is never satisfied. Perfection can always drive you crazy. I can be very cruel with myself sometimes. The ego works in a very tyrannical, data ship mode.”

Iñárritu said once he became aware of that “inner voice” concept, he thought it would be a great theme for somebody in a movie. But translating those complex thoughts isn’t exactly the easiest thing to do, as he found out.

“It took me a long time to come up with that abstract of having a presence like that which is so intense,” Iñárritu said. “It’s in all of us, but at the same time it’s very silent. Still it manipulates all of us. It’s important that it awakens you and you see it clearly, because if not, you are f—-d.”

During the long creative process of “Birdman,” Iñárritu said he had lunch in Mexico one day with Tim Burton, who prior to 1989’s “Batman” and 1992’s “Batman Returns” worked with Keaton on the classic supernatural comedy “Beetlejuice” in 1988. Iñárritu said Burton’s insights into Keaton were invaluable.

“He told me that Michael, beside his funny side, has a very, very dark side to him, and it’s true,” Iñárritu said. “He said that Michael is a very complex person, a beautiful human being and a very self-assured guy who can really navigate through drama and comedy. That’s why I wanted him, beyond the fact that he was Batman and the reality he could bring to the film. There are few actors that can bring the complex nature that this character needed. The fact that he can navigate through both genres is unique.”

While Keaton is naturally the focus of “Birdman,” the fragile states of Riggan’s fellow actors – Norton, Naomi Watts and Andrew Riseborough — are also examined in the film. And having worked with many different performers over the years, Iñárritu says actors can be very vulnerable due to the nature of the profession, if not a bit bat-s–t crazy at times, as demonstrated in “Birdman.”

“The nature of acting is very complex, because in order to be good, they can’t be themselves. It’s a very strange job to pretend to be others,” the filmmaker said. “It’s very difficult and puts them in a very vulnerable position, and they so much on others, the material and the applause. It’s a lone wolf way of living, yet at the same time fantastic. It’s a journey through their own kind of consciousness and knowledge. They have to be very perceptive, very sensitive and they have to look very deeply into life and reabsorb it. It’s complex and a little cuckoo, I guess.”