On Monday, the classic film is turning 70. With its scathing take on Hollywood mores and a comeback performance from silent-era silver screen star Gloria Swanson (“All right, Mr. DeMille, I’m ready for my close-up”), the masterpiece earned 11 Oscar nominations, including a nod for ingénue Nancy Olson, who played an ambitious studio script editor.

She had just turned 22 when “Sunset Boulevard” opened in 1950.

The Milwaukee native moved to Hollywood to attend UCLA, where she studied theater. “Sunset Boulevard” was only her second film, after appearing in the 1949 Western “Canadian Pacific.” Olson nabbed a role in the noir as Betty Schaefer, who’s engaged to assistant director Artie Green (“Dragnet” star Jack Webb) but falls for struggling, cynical screenwriter Joe Gillis (William Holden). He’s living a gigolo-type existence in a neglected Sunset Boulevard mansion, owned by delusional former star Norma Desmond (Swanson) — while, at night, sneaking out to work with Betty on his screenplay.

“Bill Holden had done several films that were huge successes like ‘Our Town’ before he left for the Army, but when he came back [from World War II] he almost had to start from the beginning,” Olson, 92, told The Post.

“He was not as confident as he might have been, and he was also in a marriage that was very rocky,” she said. “And he was starting to drink too much. Bill and I got along very well, and we began to really enjoy each other. Not romantically, although there was a moment when he somehow went over that line a little bit —but it was clear that I was not available, so that was that.”

Olson, a movie buff, said that nevertheless, when she was hired for “Sunset Boulevard,” she had never heard of Swanson — “I asked my mother who she was,” she said — but had interacted with ‘Boulevard’ director and co-writer Billy Wilder on the Paramount Pictures lot (where several scenes in “Boulevard” were shot).

“I had seen all his films and was a great admirer and in awe, and he would try to catch up with me and have a conversation as I was walking to the commissary,” she said of Wilder, who was known for his razor-sharp noir “Double Indemnity” (1944) and had won two Oscars (Best Screenplay, Best Director) for “The Lost Weekend” (1945). “It was all about what I was doing at UCLA, what it was like growing up in the Midwest, ‘I understand your father’s a doctor,’ that kind of thing. He was persistent. I never discussed this with Billy, but I think, all those talks we had, he wanted to know how I spoke, [like] did I sound like someone who could be an aspiring writer. That’s what I believe Billy was looking for.”

Olson says that Wilder, in his search for authenticity in Betty Schaefer, did not approve of how the studio initially dressed her for the part.

“[Costume designer] Edith Head, who became a great friend of mine, would do all the dressing-up stuff and put costumes on me, and Billy would look at it and say, ‘No, I don’t think so. I liked the way Nancy looked yesterday when she was visiting the set,’ ” she said. “So I ended up wearing my own clothes [in the movie] and I did not have a great wardrobe. I came from Milwaukee, and I didn’t even know where to shop.

“Billy wanted me to simply be me . . . so I understood why I was in the film.”

Olson said the cast and crew of “Sunset Boulevard” trusted in Wilder’s vision, and had an inkling during filming that it was bound to be something special.

“As we began to shoot, there began to be a buzz on the lot,” she says. “They showed dailies each night — they were making many films at once, with directors like Cecile B. DeMille [who’s in “Sunset Boulevard”] and William Wyler. Everybody would go at 6 p.m. to see yesterday’s dailies and suddenly, when we began ‘Sunset Boulevard,’ no one would leave until the dailies were shown. Everybody came to watch them. We knew that what was happening was something rare, but we didn’t realize to what extent.”

That question was answered, somewhat, when “Sunset Boulevard” was pre-screened for Hollywood big shots, some of whom took exception to its dark depiction of life in Tinseltown — and specifically, the movie business.

“There was a small screening, and [MGM co-founder] Louie B. Mayer was there,” Olson said. “I heard that after the film he came over to Billy and said, ‘How dare you do this to us?’ And Billy said, ‘Eff you’ and walked out.”

Olson made three more films with Holden — “there was such a demand for us to work together again,” she says — but wasn’t enthralled with Hollywood and scaled back her big-screen work, working regularly on television for the next 40 years (including a 2010 appearance on HBO’s “Big Love”).

She married legendary lyricist Alan Jay Lerner (“My Fair Lady,” “Gigi”) in 1950 — she was the third of his eight wives — and, following their divorce in 1957, got remarried to Capitol Records executive Alan Livingston in 1962 — a union that lasted until his death in 2009.

She and Holden remained friends — bound together forever by the magic of “Sunset Boulevard.”

“Years ago, when I was married to Alan Livingston, we were in JFK Airport on our way to London,” she said. “And Bill was coming in from Europe. We hadn’t seen each other in a couple of years and we were rushing to get to the plane, and I heard ‘Nancy! Nancy!’ and I turn around and at the end of the hall is Bill Holden.

“We turned and ran toward each other and embraced,” she said. “A man walked by and, I’ll never forget this, he tapped us on the shoulder and said, ‘This is better than watching an old movie.’ ”