Irish actor Brendan Gleeson spent nearly 14 years deciding to be an actor. He was never sure he could cut it and didn’t begin until he was 34 

“I was sure I wouldn’t have been that sort of person,” he says in a meeting room here. “I wasn’t aggressive and proactive, except when I was teaching school, for example. I’d put on plays, and I’d bring them off because I found, well, that’s what I SHOULD be doing. But with regard to my own stuff, I didn’t believe I’d have that self-motivation.”

Gleeson, best known for “In Bruges,” “Troy,” “Cold Mountain,” and as Mad-Eye Moody in the “Harry Potter” films, is testing his mettle again in AT&T’s original series “Mr. Mercedes,” airing on the Audience Network.

Gleeson plays a retired police detective who is tormented by an anonymous serial killer in the spooky series based on Stephen King’s trilogy. Gleeson says he was seduced by “Mr. Mercedes.”

“It was really about the writing, the casting, and about how vital it was going to be,” says Gleeson, stroking his gold-and-white beard. “The integrity it had for a start, but also how much vitality and vigor it had and how much attention to detail. I was really surprised by it. Once it took off, I knew I was in good hands.”

Not finding “good hands” was an abiding fear for Gleeson, who spent 10 years teaching English and Gaelic. “I did a lot of little bits and bobs trying to find a place in the world, I suppose. I hated working in offices, the confinement of it. I was working at the airport in a cargo transfer customs thing. And I found it very exciting and new and bright for about three months.

“And then I got double the wages to go into this health board, and I was thinking, ‘This is great. I’ll work here for a couple of months and then I’ll go back to college.’ I ended up staying two and a half years. And I remember feeling I was dying slowly. I just couldn’t live with it. It was so soul destroying. So I went back to college.”

He was older than the other students and had to readjust to poverty again. That didn’t sit well. “Then I began acting and saying, ‘Oh, this is something I can do.’ But there was a time there I didn’t know what the hell I was going to do. That was about a good 10 years I didn’t know what the hell I was going to do.”

His wife encouraged him to try performing full time. But even then, he started slowly. “I’d met these guys in college and they started a theater company that began to address theater in the suburbs and write about subjects in the suburbs, the people at home. There was a lot written about the city center, a lot written about the country, but actually most people live in the suburbs …

“So we put on a lot of shows and got a bigger and bigger audience all through the ‘80s. And we started going into the mainstream theaters, went into the Olympia and filled it for 10 weeks. So it became obvious that something was going right. I got nominated then for a theater award, even though I wasn’t full time. And I began to think, ‘What do you need to know? At what point are you going to take it on?’”

But Gleeson was still apprehensive. “I had a very odd notion of what the world (of acting) was about, really. There’s a lot more idealism and a lot more positivity in it than I gave it credit for, I think. I kinda felt I was suspicious of it, that I would have to do things I didn’t want to do in order to make a living,” he says in his Irish brogue.

“And I never wanted to do that. It was kind of too precious. I liked to be able to keep it at arm’s length where I could do whatever I wanted on my own time. But it became obvious to give it a proper shot, I needed to commit to it.”

Finally, he did commit and has been working steadily ever since. He and his wife of 35 years reared four sons, two of whom are actors.

Gleeson thinks he hasn’t changed since he fabricated trendy leather goods for hippies and languished away in business offices. “I’m essentially the same person if I’m honest,” he says.

“I’ve tried to keep the child alive, really. And this is a good business to be able to keep the child alive in. And that has to do with curiosity, maintaining curiosity throughout the world. But also it’s a disappointment. There are times when you don’t see the world as a particularly attractive place. But for the most part, I think I’ve been lucky enough to keep curiosity in place and to keep a sense of joy there.”