TORONTO – He may have co-written one of the most popular musicals in the history of theatre. But that doesn’t mean Alain Boublil is immune to problems at customs.

“They had a random computer that said, ‘Number 49, send him to the office,’ and I was number 49,” Boublil, 72, said recently at a Toronto rehearsal space.

“The guy looks at me, ‘Where are you coming from?’ very, very nastily, and he said, ‘What do you do?’ … And I said, ‘musical theatre,’ and he said, ‘OK, what did you write?’ and I said, ‘”Les Miserables.”‘

“And the guy suddenly freezes, and says, ‘It’s an honour, sir,’ folds the paper, and says, ‘Thank you, have a good stay.’”

If musical theatre had a Mount Rushmore, it’s likely that Boublil and Claude-Michel Schonberg — the famed French collaborators who were both in town to assist with the upcoming Mirvish remount of “Les Miserables” — would find their likenesses on it.

The show is one of the world’s best-selling, growing from its first production in Paris in 1980 to be seen by millions of people across 42 countries, with its 2012 film adaptation directed by Oscar winner Tom Hooper winning over box offices; they’re also renowned for their collaboration on the smash hit “Miss Saigon.”

And it’s clear there’s still passion for their work: Just last week, the London remount of “Miss Saigon” brought in more than $7 million in box office sales, breaking the all-time record for the largest single day of sales in West End and Broadway history.

But despite their impact, these are clearly two men who relish the kind of relative anonymity afforded to auteurs that would still, for example, make them wait at customs.

“(Anonymity) is not something I like — it’s something I love,” said Schonberg, 69. “What I enjoy is that we have the fame, but not the popularity. It’s fantastic to have something well-known in the world, but when you walk on the street, people don’t grab you and say, ‘This is that guy.’ I hate that kind of thing. For the moment, we have the best of both worlds.”

Boublil recalls that people said Victor Hugo’s 1862 epic of love against the bloody backdrop of Paris’s June Rebellion couldn’t be set to music — even legendary opera composer Giacomo Puccini tried and failed, he said.

Still, decades after proving the belief decidedly wrong, the pair remain bemused by the musical’s success. Boublil even credits outside forces — Susan Boyle’s viral 2009 performance of “I Dreamed A Dream” and the 2012 movie — for giving “Les Miserables” this second wind.

“What I’m surprised about is that the show is still there,” said Schonberg. “In our wildest dreams we never imagined we’d be there 28 years after. So from this point, anything is possible. To have written the most popular show ever, in the world, that’s something that surprises me every day.”

But Boublil and Schonberg are old-school theatremakers, and tinkering with their scripts, they say, is a musical theatre tradition. They are currently working on a “significant” rewrite of their 1996 play “Martin Guerre,” and even “Les Miserables” undergoes nips and tucks.

“There are no famous shows,” said Schonberg. “Musical show is (a) living art form. It is not frozen, like a movie can be. It was a tradition in musical theatre, and every time there was a new location, there would be rewriting.

“The show has been evolving slowly,” added Boublil. “But there has always been a little scene to add, to modify, and we enjoy it immensely when something is not definitely set in stone too early.”

The pair worked on the film’s screenplay, and one of the more notable changes they have planned for the Toronto remount was inspired by the movie.

“I suggested maybe ‘I Dreamed a Dream’ would come after Fantine would meet her first client, rather than after being fired from the factory, which was what has been done for 25 years in the stage play,” said Boublil.

“Apparently, that worked very very well, and we are trying to implement that in the show. But it is not easy. You cannot see with one (zoomed-in) image see the distress on the face. Here we have to rebuild the whole five minutes before and 10 minutes after to make it work.”

But for the stage veterans, the film adaptation provided new terrifying frontiers. For years, said Boublil, a film would mark the end of the stage show and, if the movie struggled, the musical would suffer too.

“The movie accelerates everything. If it kills you, you’re dead, and it’s finished and you never come back,” he said.

“But what’s wonderful is that the movie has brought and is bringing to the stage play a whole new generation of people, especially between (age) 10 and 15, which is unheard of. We didn’t create (this trend) — it started a few years ago, when ‘Chicago’ survived the movie, and many others.”

“Today, it looks like the movie is the coronation of a stage show,” added Schonberg. “It makes it officially a classic, a standard. A standard by definition is something people go, see, come back, and see again and again and again.

“But we are always scared. That is our life, it’s to be scared. And we would never do something when we are not scared. What’s the interest in doing something if it’s going to work really well?”

For all their love of risk, however, the two — who have known each other since 1967 — benefit from knowing each other very well, and share a familiarity that extends beyond their work. When the interview ends, Boublil leaves to make a call, and returns, nodding at Schonberg — these two giants of their field, with two massive contributions to their industry — and asks him, simply, in his native French: “Will you dine with us tonight?”

He agrees. Boublil makes a reservation, just a table for three.

The Mirvish remount of “Les Miserables” will run at Toronto’s Princess of Wales Theatre starting Sept. 27.