Entertainment

When ‘Rain’ was plain

Singin’ in the Rain,’’ which arrives on Blu-ray in celebration of its 60th anniversary on Tuesday, is widely regarded as the best musical of all time by both critics and the public. Virtually no one in the world is unfamiliar with the image of Gene Kelly joyously dancing the title song on a rain-swept Hollywood sound stage.

All that glorification was unimaginable when the musical was being shot in 1951 on the backlot at MGM, which had just seen its prestige musical “An American in Paris’’ (also starring Kelly) sweep the Oscars.

“Nobody had the slightest idea it would someday be listed among the greatest films of all time,’’ co-star Debbie Reynolds tells The Post. “We thought it was just another big, splashy MGM musical.’’

“Only [screenwriter] Adolph Green thought it would hold up’’ over time, says Gene Kelly’s widow Patricia. Green and his partner, Betty Comden, had been tasked with concocting what we now call a “jukebox musical’’ to showcase songs producer Arthur Freed had written in the late ’20s and early ’30s with Herb Nacio Brown.

What the writers came up with was far more memorable than the wafer of a script for “An American in Paris’’ — an affectionate, remarkably accurate depiction and extremely funny comedy about Hollywood’s scramble to adapt to the coming of sound.

It’s one of a handful of classic movies that can be called both timeless and perfect — and that especially includes the casting. Tap dancer Donald O’Connor, a brilliant comic improviser, was tapped by Kelly and co-director Stanley Donen to join Reynolds in the cast.

Comden and Green wrote the part of Lina Lamont — a silent-screen diva whose screechy voice is doubled by Reynolds — for their friend Judy Holliday, but she became too big a star in “Born Yesterday’’ to be given the supporting role. It went to Jean Hagen, a contract player at MGM known for dramatic roles who walked off with one of only two Oscar nominations nabbed by “Singin’  ” (the other was for scoring) for the movie’s funniest performance.

In a new 50-minute documentary on the spectacular-looking Blu-ray release, contemporary musical directors like Rob Marshall (“Chicago’’) and Adam Shankman (“Rock of Ages’’) marvel at the numbers, photographed in long, uninterrupted takes showcasing the dancers, including Cyd Charisse, photographed from head to toe.

This was possible because Freed’s production unit had the money and freedom to rehearse for months and to spend weeks shooting a number until it was perfect. “There was minimal editing then,’’ says Shankman. “Now you have to edit around mistakes.’’

Part of the joy of “Singin’ in the Rain’’ is that hard-working perfectionists like Kelly and O’Connor make it all look easy — which it certainly wasn’t when a soaking-wet Kelly had to shoot the intricate title song-and-dance while suffering a 103-degree fever.

Among the film’s millions of young fans are cast members from “Glee.’’ In the documentary, they demonstrate just how difficult it was to restage even part of O’Connor’s famous “Make ’Em Laugh’’ number — in which he runs up a wall, does backflips and crashes into the scenery, among other things — for the TV series.

When the new digital restoration debuted in April at the TCM Classic Film Festival in Hollywood, Reynolds recalled chewing gum while rehearsing the “You Were Meant For Me’’ number while sitting on a prop ladder. When shooting began, she absent-mindedly stuck the wad of gum to the ladder.

But when Kelly’s head brushed next to the ladder while he was dancing, the gum “stuck to his [toupee] and pulled it off,’’ she says. “He was mad at me for a year after that.’’