Late in “The Sound of Music” production – after a helicopter’s downdraft had knocked Julie Andrews over, after rain clobbered the schedule and budget, and after an angry farmer poked holes in a man-made brook – the leading lady delivered a tentative prediction.

“This smells as if it might be a success,” Andrews said.

The 1965 musical was so phenomenally popular that it was nicknamed “The Sound of Money” and dubbed “The Mint.” The movie played in American theaters for four and a half years until the studio withdrew it with plans for a re-issue; the demand in England was so insatiable that the film set a record with 170 weeks at a London theater.

Decades later, while promoting a live CBS presentation of “On Golden Pond” with co-star Christopher Plummer, Andrews said, “I don’t think either of us knew it would take off that way. … I guess when you put all those ingredients – beautiful scenery and beautiful music and children and nuns and all of that – together, the only thing that was missing was Lassie, I guess.”

Who needs Lassie when you have nearly 2.5 million Facebook followers and a looming golden anniversary that will revisit the glories of the musical?

For starters, Lady Gaga performed a medley from “The Sound of Music” at the Feb. 22 Oscar ceremony and was greeted by Andrews afterward.

The restored film will return to 500-plus theaters April 19 and 22, and Twentieth Century Fox Home Entertainment will release a five-disc edition March 10 with a new documentary, “The Sound of a City: Julie Andrews Returns to Salzburg.”

The movie also will celebrate its golden anniversary by opening the TCM Classic Film Festival March 26 at the TCL Chinese Theatre in Hollywood, where Andrews and Plummer are expected for a rare reunion.

A half-century ago, preview audiences accurately predicted how most of the world would embrace the movie. After test runs in Minneapolis and Tulsa, 360 audience members rated it excellent, five called it good and no one labeled it fair.

It premiered March 2, 1965, at the Rivoli Theatre on Broadway in front of an audience that included Richard Rodgers, Bette Davis, Salvador Dali and Adlai Stevenson, Richard Stirling’s biography of Andrews reported. Davis told the leading lady, “The motion picture business is in love with you!”

It began as a “roadshow” engagement, with reserved seating at a very limited number of theaters, higher prices ($1.50 to $3 depending on the day, time and whether you were in the nosebleeds) and scheduled film start times instead of continuous shows. It also had an intermission, just like a Broadway play.

That is the opposite of the release pattern today, where a movie often will open on as many screens as possible and aim to be the box-office champ for the weekend or longer. A popular film might hold for a couple of months before starting its speedy march to DVD, Blu-ray and on-demand viewing.

“The Sound of Music” was an event, and even by Dec. 18, 1965, it was playing in only 131 of the 14,000-plus theaters in the United States. The movie musical broke records in 80 percent of those U.S. locations and a Fox vice president famously pointed to Salt Lake City as an example of its success. The city’s population then was 199,300 and yet a theater there sold 309,000 tickets.

For a time, “Sound of Music” displaced “Gone With the Wind” as the domestic box-office champion of all time, although it now stands at No. 3 when adjusted for inflation, behind “GWTW” and “Star Wars.”

As for its enduring appeal, Barry Monush, author of “The Sound of Music FAQ” (Applause Theatre & Cinema Books, being published March 10), has no easy or single answer.

“I adore the film. I meet people all the time who adore the film, and I meet just as many people who just scowl at the very mention of the name,” he said in a recent phone call.

Monush, whose day job is assistant curator at the Paley Center for Media in New York, first saw the movie as a 6-year-old at a theater in Asbury Park, N.J. It was his sister’s 10th birthday in October 1965, and an aunt had an extra ticket, and because it starred the actress from “Mary Poppins,” he was thrilled to tag along.

He didn’t really understand who the Nazis were, but he knew they were menacing.

“I loved it in 1965 and I still love it,” he said. “Every time I see it, it still elevates me.”

As for why it took the world by storm in 1965 and forever after, he’s not quite sure. “We’ve all seen movies that we find absolutely wonderful that don’t make anywhere near as much money and don’t endure as well.”

It has a great score, but so do other films that don’t make a cent.

“It is extremely good storytelling. I mean, I think Robert Wise deservedly won that Academy Award for best director,” making a nearly three-hour movie that flies by. “And the casting of Julie Andrews is pivotal, it’s just pivotal. … Somehow she manages to make that character so real and never cloying. I really think she’s an essential part of why people fell in love with the movie.”

AS THE 50TH anniversary approaches, some other reasons why it flourished:

• Many baby boomers remember getting dressed in their Sunday finery and traveling to an old-fashioned movie palace with their family to watch the picture advertised as “The Happiest Sound in All the World.”

Even if, as a jaundiced adult, you think of it as treacle, you cannot deny your childhood affection for it. And, if you come across it on television, you may not watch the entire movie but you might stick around for “Sixteen Going on Seventeen” or “Edelweiss.”

It has everything from a basis in fact and a fairy-tale romance to soaring Rodgers & Hammerstein music, spectacular scenery, thrills, love, the chance for seven children to have a mother and their father a mate again, terror and, ultimately, triumph over the Nazis.

Timing is everything – Wise speculated that volatile 1965 provided the perfect launching pad.

“Newspapers carried headlines of the war in Vietnam, a cultural revolution was beginning to spread throughout the country, and people needed old-fashioned ideals to hold on to. The moviegoing public was ready, possibly even eager, for a film like this,” the director wrote in a foreword to “The Sound of Music – The Making of America’s Favorite Movie” by Julia Antopol Hirsch.

“Besides an outstanding score and an excellent cast, it had a heartwarming story, good humor, someone to love and someone to hate, and seven adorable children,” he added.

•  Before the movie, “The Sound of Music” was a Broadway musical. It opened in November 1959, earned star Mary Martin a Tony over Ethel Merman in “Gypsy” (“How do you buck a nun?” the also-ran asked) and played for 1,443 performances.

The president of Twentieth Century Fox had seen the musical on opening night and reportedly was moved to tears. In June 1960, the studio bought the film rights for $1.25 million, against 10 percent of the gross.

•  One reviewer suggested “Sound of Music” would restore filmgoers’ faith in movies and even humanity, but many blasted it as saccharine, corny and old hat. The New York Times suggested most of the adult performances, including Plummer, were “fairly horrendous.”

Nearly three decades ago, George Anderson, the Pittsburgh Post-Gazette’s Magazine editor, summarized it this way: ” ‘The Sound of Music’ is, ah, you know. The movie intellectuals love to hate, that the innocent love too much. Still, there is something there to make it endure like this. Watching it, you never quite believe it. But you watch it.”

•  No one dubbed Andrews’ musical numbers. Her voice was glorious.

In 1997, she had noncancerous nodules removed from her throat that unexpectedly cost the singer her four-octave range, making the movie and its soundtrack all the sweeter and more prized.

•  There really was a Maria von Trapp, a onetime postulant who was sent to the home of retired naval captain Georg von Trapp and his brood.

She and the widower were married in 1927, their three children joined the seven from his first marriage, and they escaped from the Nazis. The stage musical and then film arrived long before the Internet turned critics and patrons into truth squads ferreting out differences between fact and fiction.

The real Maria von Trapp said she had been a wild creature. “Julie Andrews and Mary Martin were too gentle – like girls out of Bryn Mawr.”

•  Nominated for 10 Academy Awards, it won five, for best picture, director, film editing, music and sound. The nominations also included Andrews for best actress and Peggy Wood, as Mother Abbess, for supporting actress.

•  A woman in Wales, Myra Franklin, earned a spot in the Guinness Book of World Records for watching the movie on the big screen 940 times. Hirsch’s book reported that a Denver truck driver saw the film from the same seat in the same theater every Sunday for three years and then, when the theater closed down, bought the seat.

•  Re-releases, special TV airings, videocassettes, DVDs and singalongs in which audience members dressed up like the characters and crooned along fed the appetite.

Fox’s new five-disc ultimate collector’s edition will have more than 13 hours of bonus content and the new documentary in which Andrews returns to the Nonnberg Abbey, Church of Mondsee where the screen wedding was staged, and the “Do-Re-Mi” steps of Mirabell Gardens.

It also will feature a 50th anniversary soundtrack from Legacy Recordings/Sony Music with eight bonus tracks never released in the States. Also due is a special anniversary soundtrack featuring the full film score.

A partnership with Princess Cruises will mean the movie will screen during multiple sailings throughout the year. In addition to Monush’s book, three others are due: “The Sound of Music Companion – 50th Anniversary Edition” by Laurence Maslon; “The Sound of Music Story” by Tom Santopietro; and “The Sound of Music: BFI Film Classics.”