Dammit, Janet, you’re 45!

A science-fiction, B-movie-style musical, “The Rocky Horror Picture Show” opened Sept. 25, 1975, and became the king of cult hits after the Waverly Theater (now the IFC Center) in Greenwich Village began showing it at midnight in 1976. But these were no normal screenings. Raucous, foul-mouthed viewers were yelling “A - - hole!” at the bespectacled hero Brad Majors, played by Barry Bostwick, and “Slut!” at virgin heroine Janet Weiss, a young Susan Sarandon. Many of the funny shouts are, ahem, unprintable.

That wild mood led to average Joes dressing up in leather corsets and chunky heels, or stripping down to lily-white skivvies, and bringing prop newspapers, gloves and flashlights. The trend expanded to hundreds of theaters and college campuses worldwide. And when cinemas reopen, fans will continue the tradition — more than four decades later.

“I love that audiences have derived such pleasure from it,” director and co-writer Jim Sharman, 75, told The Post from his home in Australia. “Whether you dismiss it as B-movie schlock — which some of it is, often deliberately so — or perceive its depths which simmer below the surface, or treat it as wallpaper for a party, it seems to satisfy some mysterious need.”

I would like, if I may, to take you on a strange journey . . . to the origins of “The Rocky Horror Picture Show.”

The musical started out in 1972 as little more than an idea for an intimate rock show. Sharman was an Australian theater and film director living in London, who had a hit with the West End production of “Jesus Christ Superstar.” At the time, Sam Shepard had asked Sharman to direct his play “The Unseen Hand” there, featuring a British actor named Richard O’Brien as an alien.

“Richard showed me a sketch, about 10 pages from memory, called ‘They Came From Denton High’ and played me the opening song ‘Science Fiction/Double Feature’ accompanying himself on guitar,” Sharman said. “It chimed with my interests. We were a generation brought up on sci-fi and late-night movies, which we loved. And after suggesting a name change — it was a rock ‘n’ roll horror show, so why not say what it is, was my logic — it developed into ‘Rocky Horror.’ ”

Sharman secured the upstairs space at the Royal Court Theatre, and O’Brien would not only compose the rockin’ songs such as “Time Warp” and “Touch-A, Touch-A, Touch Me,” but play the long-haired handyman, Riff Raff.

“Rocky Horror” was shaping up, but it had an itch to scratch and needed assistance. The perfect actor had to be found to play Dr. Frank-N-Furter, the sweet transvestite who hosts the annual Transylvania Convention at his foreboding castle. Thankfully, a then-no-name actor arrived — Tim Curry.

“Tim auditioned singing Little Richard’s ‘Tutti Frutti,’ ” Sharman said. “He ripped the Royal Court roof off. He had it, then and there . . . and, beneath the makeup, Tim was that rare thing, a thinking actor with an adventurous streak, and [had] the technical chops to really deliver. He simply got it.”

“The Rocky Horror Picture Show” opened in 1973 and was a hot ticket overnight.

“We really thought we were doing an indie musical for three weeks at the Theatre Upstairs on no budget and minimum fees for the love and anarchy of it,” Sharman said. “But when rock stars and famous artists were fighting for tickets, it sunk in.”

Film and record producer Lou Adler saw the show when it transferred to a larger theater. Adler was a huge name, having produced Carole King’s “Tapestry,” and wanted to stage the show in LA and then make a film of “Rocky Horror.” He believed in the material, and let Sharman and O’Brien run wild.

“Beyond a cut to the final song in an early version, quickly reversed following some fan complaints, to my knowledge, there was little to no studio interference in the movie,” Sharman said. “By agreeing to a very low budget and a very short schedule, which guaranteed a lot of the original creatives their rightful role in its making, it slipped under the radar.”

Curry and O’Brien reprised their iconic roles on-screen, joined by Sarandon, Bostwick and Meat Loaf as Eddie. There were also some added duties for the composer. Onstage, the opening song “Science Fiction/Double Feature” was sung by a female usherette, but that fateful 1972 meeting with O’Brien lingered in Sharman’s mind for years.

“I never forgot Richard’s haunting, yet slightly androgynous, voice singing ‘Science Fiction’ the first time I heard it, so I thought he should sing it,” Sharman said.

O’Brien wouldn’t just stand still and perform, though. Sharman wanted lips. Big red lips. Inspired by the surrealist films of Luis Buñuel and a Man Ray painting of a floating mouth in production designer Brian Thomson’s office, Sharman decided he had to have smackers and teeth.

“There were a lot of mouths and lips floating around at the time!” he said.

The director had also had a dream one night in which “Riff Riff and Magenta somehow coalesced into one androgynous being.” This is why, in the film, we see actress Patricia Quinn’s lips and hear O’Brien’s sky-high voice. Like the rest of the movie, that mouth has stayed an iconic image in pop culture.

Today, Sharman is hardly a one-hit wonder, having remained a prolific theater director in Australia. Despite working often, he looks back fondly on the “mental mind f - - k” he gave the world.

“I love that outsiders have found a movie that speaks to them. It’s certainly the late-night audience who made it a classic,” Sharman said. “It wasn’t made like any other movie and was left alone to be itself, so it has a life of its own.”