Leonard Bernstein pal reveals secrets of Bradley Cooper biopic

Over the past few weeks, Bradley Cooper has been seen in New York wearing a retro suit and a prosthetic nose for his latest role as Leonard Bernstein, for the upcoming Netflix film “Maestro.”

Now, a former protege and friend of the music legend is revealing private stories that the Oscar-nominated actor has likely never heard about the composer of “West Side Story” — including how the married Bernstein slept with his protégé’s boyfriend.

Conductor Mark Janas, 70, was friends with Bernstein beginning in the mid 1970s until the composer’s death in 1990, and last month he managed to get a letter to Cooper, through the actor’s, Hollywood agent detailing some of his stories of “the great maestro,” which he shared exclusively with The Post.

Cooper, 47, is directing and starring in the movie, scheduled for release next year.

Janas was 23 years old and an aspiring conductor when he met Bernstein, known to friends as Lenny, in the summer of 1975.

“I was a bit unprepared for Lenny answering the door wearing a Speedo, flip flops and a white terrycloth bathrobe, with a glass of Scotch in one hand and a joint in the other,” Janas wrote in the letter he sent to Cooper and shared with The Post.

The conductor, then 56, was already established as one of the world’s greatest musicians but had left his position as the longtime music director of the New York Philharmonic to focus on composing. Although he lived with his family in a sprawling eight-room apartment at the Dakota on Central Park West, he was staying that summer at the lakeside cottage of his manager Harry Kraut in the Berkshires.

After Janas — a musical prodigy who began playing piano at 3 and composing music at 5 — nearly beat the maestro at the celebrity-guessing-game Botticelli, Bernstein invited him to rehearsals at Tanglewood, the Boston Symphony Orchestra’s summer home.

Bernstein nicknamed Janas “Chico” because he thought Janas resembled a young version of comedian Chico Marx.

That summer they spent hours discussing music, and one day, at the grand piano in Bernstein’s Tanglewood dressing room, Janas began to play Dmitri Shostakovich’s Fifth Symphony. Janas described the Soviet composer’s second movement as an elegy, “in this case, for his creative freedom after artistic censorship by the Soviets” who had punished him for becoming too avant-garde. Bernstein was deeply impressed by the analysis, and on August 9, 1975, when news broke that Shostakovich had died, Bernstein used Janas’ interpretation of the Fifth Symphony during a concert performance in Amsterdam.

“He told me that he remembered our conversation and that night he programmed the movement into his concert at the Concertgebouw to honor the composer,” Janas told The Post. “I can’t even tell you how that made me feel. My eyes still well up whenever I think of it.”

Months later, Janas found himself working as Bernstein’s assistant at the Boston Symphony and the New York Philharmonic. In 1977, Bernstein asked the Indiana University undergrad to conduct festivals of his music in Israel, Austria and Yugoslavia.

In 1982, after Janas completed his masters degree in orchestral conducting at Rice University in Houston, he found himself in the audience at Jones Hall while Bernstein conducted Tchaikovsky’s “Francesca da Rimini” with the Israeli Philharmonic.

“The audience had gotten a bit restless during the final stretch of the relatively unknown work,” he said. Suddenly, Bernstein fell from his conductor’s podium, and while he was being helped back up by the principal cellist and concertmaster, he was “shouting at the top of his lungs, ‘Keep going!’” He repeated it in Hebrew, Yiddish, German and Russian, said Janas, and the audience was on the edge of their seats.

“it is impossible to communicate what it was like hearing the musicians sounding like they were somehow fighting a war and running from a wildfire right through the coda to the very end of the piece,” he said. “The ovation was deafening.”

While Janas worshipped the man who he called the most documented musician of all time, he was also clear-eyed about some of his shortcomings. At one point, Bernstein, who was married with three children, had an affair with Janas’ boyfriend. Bernstein’s homosexuality was well known although he had promised his wife to keep his dalliances discreet.

“You are a homosexual and may never change,” Bernstein’s wife, Felicia Montealegre, wrote in a letter after their 1951 wedding. “I am willing to accept you as you are.”

“The maestro’s position on his pedestal was unassailable — after all, it was I who put him up there,” said Janas, referring to his own betrayal by the conductor. “But now I could spot clay feet for the first time.”

Still, Janas continued to be inspired by the mercurial conductor. “There’s a small photo on a shelf above my piano: Lenny, conducting, his head back, eyes closed, absorbed in a moment of musical bliss,” he said. “If ever I felt inspiration or energy waning, Or … fear or self-doubt, all I had to do was look up and Lenny was there to remind me that, in the end, it’s all poetry and passion. You have to give in to it.”