Michael Keaton's “Knox Goes Away ”Trailer: Star Directs Himself as a Hit Man with Weeks to Live (Exclusive)

The 'Batman' and 'Beetlejuice' icon tells PEOPLE why he was "somewhat frightened" to direct the dramatic thriller

Michael Keaton pulled double duty on his new thriller Knox Goes Away.

The Batman and Beetlejuice icon, 72, directed the film while also starring as the titular character, John Knox, a principled hit man who lives in what Keaton calls a moral "gray area" — i.e., he kills people, but only bad guys.

PEOPLE has the exclusive first trailer, which shows Knox's doctor diagnosing him with a rapidly-advancing form of dementia. "How much time do I have?" John asks the doctor, who replies, "Weeks, not months."

John takes the news in stride, with a bit of gallows humor too: "Even if I hated you for telling me," he replies. "I'd forget soon enough."

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His remaining time alive is stressful: He botches a hit job and also assists his estranged son Miles (James Marsden), who's in enormous trouble. The younger Knox, bloodied and rattled, shows up at his dad's door after he stabbed a "bad man" who took advantage of Miles's teenage daughter.

John, in attempt to make amends, agrees to come to Miles's rescue. It's not long, though, before dogged detectives begin to close in.

Though Keaton plays a contract killer, he doesn't consider Knox Goes Away a "hit-man movie," he tells PEOPLE.

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"There’s been about 9,000 hit-men movies made, and everybody's done it as good as you can do it," Keaton says. "I had zero interest in making a hit-man movie. It isn’t and never was."

"I'm in the adult movie business," Keaton jokes, meaning the movie tells the story of the complicated relationship between a son and a father — one who's facing his own mortality.

Keaton made his feature-length directorial debut in 2008 with The Merry Gentleman, and Knox Goes Away marks the first time since that he's back behind the camera.

What took so long? "I’ve had offers and opportunities, but mostly the problem was what an undertaking it is. When would I find the time to do it?" he says.

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"It has to be something you really want to make, a good solidly written script," he says, before quipping, "If there’s a really great one, there’s the likelihood Christopher Nolan or Steven Spielberg are gonna beat you to it."

But he said yes to Knox Goes Away years ago, then kept putting it off. "I was somewhat frightened to do it," he admits. "I thought, 'I got to get this right.' "

He did — and under tight time constraints too: Keaton had only 25 days to shoot the film. "Every day was pressure," he recalls. Turns out, he's grace under fire. Keaton notes he wrapped filming in 24-and-a-half days.

Knox Goes Away, which costars Al Pacino, Marcia Gay Harden and Suzy Nakamura, is in theaters March 15.

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