In his 25 years playing quirky Belgian detective Hercule Poirot on television, British actor David Suchet thought he had seen it all. Until one day this summer while he was performing in a play in Los Angeles, when he went backstage and found a 12-year-old boy waiting for him dressed exactly like the character of Poirot, down to the dapper hat and cane.

“I mean, a 12-year-old,” Suchet, 68, marveled a few weeks later by phone. “I said to him, ‘Would you like me to sign your hat?’ and you’ve never seen anybody so happy in your life. It was so moving to me. It was very humbling.”

The only thing surprising about that story, really, is that Suchet seemed surprised at all. “Agatha Christie’s Poirot,” based on Christie’s popular character from her mystery novels, has commanded an extremely loyal international fan base – of all ages – since 1989. Now, as “Poirot” gets ready to conclude with its final three episodes (available exclusively on Maryland-based subscription streaming service Acorn TV, rolled out weekly starting Aug. 11), millions of viewers are already mourning the loss of their beloved detective.

While Poirot has been portrayed by other actors through the years in various productions, none have captured the detective’s eccentricity and dry humor quite like Suchet in the long-running U.K.-based TV series, which aired on PBS and in multiple languages worldwide. He threw himself into the character, methodically mastering his walk and accent – and that epic mustache. Time and time again, Poirot used his special investigative skills and keen eye for detail to crack many different cases.

It’s rare that an actor gets to play the same character for a few seasons, let alone for decades. Of course, the show was produced on the U.K. television timetable, so there were fewer episodes (70 total), and sometimes that meant there were year-long gaps between seasons, which allowed Suchet to pursue other projects. But Suchet and the producers were fastidious about making sure they stayed true to Christie’s story.

Suchet recalled that the most challenging aspect of playing the character for decades was simply not to evolve, as Christie didn’t provide many changes in the books. Tiny things were altered (like the width of stripes on his trousers, or wearing a wristwatch instead of a pocket watch). Except for the natural aging process, Poirot stayed largely the same.

“It’s not only the inclination of the actor, but also the inclination of the director to come in and make him a joke-y figure,” Suchet said. “So I had to hang on to my own integrity to make sure I kept what Agatha Christie wrote rather than something that someone else might think may be more entertaining, or would get a bit of a laugh.”

That was especially crucial. And as Suchet found out, if viewers think something on the show strays too far from the novels . . . well, he would hear about it.

Suchet made it his goal to answer as many of those disappointed letters as possible. “I’ve written back sympathizing with them, but also defending our positions by saying that whenever possible, we’ve filmed accurately,” he explained. “But when the writers deemed that the stories wouldn’t transfer to the screen the way they did in the novels, then they had to take liberties.”

“I’ve always apologized for that,” Suchet added. “It’s a fair comment.”

So after 25 years of answering the same questions, he’s really that polite? Oh yes – Suchet still takes his responsibility as the reigning Poirot very seriously, from responding to fan mail to chatting with people on the street when they recognize him to answering many questions about his favorite episodes. (“Murder on the Orient Express,” “Death on the Nile” and “The ABC Murders” are at the top of his list.)

He realizes that because the show has run for so long, people have formed a deep bond with the series. And they feel particularly protective of Poirot, a “safe” character that always makes an effort to always be kind. In some ways, that’s unusual to find nowadays, especially with the anti-hero complex found in so many television shows in the mystery genre.

Now, as he signs off, Suchet admits he’s going to have trouble shedding this character that has been part of his identity for more than a third of his life. When asked what he’ll miss most about playing the detective, it’s an overwhelming question.

He paused to think about it. “I think I’ll miss him,” Suchet said. “He became much more than just a character. He became my best friend.”

The last three episodes of “Agatha Christie’s Poirot” will be available on subscription-based streaming service Acorn TV (www.Acorn.TV): “Elephants Can Remember” (Aug. 11), “Labours of Hercules” (Aug. 18) and “Curtain: Poirot’s Last Case” (Aug. 25). The episodes will be syndicated to PBS stations in November.