Written By: Ben Cosgrove

You would think that Marilyn Monroe is such an object of fascination that every photo of her would have surfaced ages ago—if not during her life, then at least soon after her death in 1962.

But in 2013, a book by Christopher Andersen, These Few Precious Days: The Final Year of Jack With Jackie, made news because it asserted that Marilyn Monroe actually phoned Jackie Kennedy in 1962 and told her that JFK was going to make her, Marilyn, his second wife. That spurred LIFE.com to dig into its archives and find a series of pictures that LIFE’s Loomis Dean made in 1948, when Marilyn was a mere 21 years old. None of Dean’s photos from that shoot had ever been published in LIFE.

So. Here she is, with another then-aspiring actress, Laurette Luez, and Hollywood veteran Clifton Webb on the set of a comedy called Sitting Pretty. Neither Marilyn nor Luez were in that movie. But Luez was under contract to Twentieth Century Fox—the studio that released Sitting Pretty—and Marilyn had once been under contract to Fox, and eventually would be again, so the presence of the two women on the set, whether as young actresses looking for pointers, or for publicity purposes, isn’t all that surprising. In fact, as Marilyn and Laurette Luez change seats at one point (the fourth image), it’s highly unlikely that these are purely impromptu shots of the trio.

(Incidentally, Webb was nominated for a Best Actor Oscar for his role in the film, one of three Academy Award nods he earned in his long career.)

It’s always jarring to see Marilyn as, in effect, an ingenue, in the years before she achieved stardom and then went on to transcend the movies and enter a realm of tragic legend. But in early 1948, Marilyn Monroe was just another talented, engaging young actress who hoped to be famous someday.

Be careful what you wish for.

Then-unknown actress Marilyn Monroe with Clifton Webb and Laurette Luez on the set of a 1948 comedy, "Sitting Pretty."

Then-unknown actress Marilyn Monroe with Clifton Webb and Laurette Luez on the set of a 1948 comedy, “Sitting Pretty.”

Loomis Dean/Life Pictures/Getty Images

Then-unknown actress Marilyn Monroe with Clifton Webb and Laurette Luez on the set of a 1948 comedy, "Sitting Pretty."

Marilyn Monroe, Clifton Webb, Laurette Luez, 1948

Loomis Dean/Life Pictures/Getty Images

Then-unknown actress Marilyn Monroe with Clifton Webb and Laurette Luez on the set of a 1948 comedy, "Sitting Pretty."

Then-unknown actress Marilyn Monroe with Clifton Webb and Laurette Luez on the set of a 1948 comedy, “Sitting Pretty.”

Loomis Dean/Life Pictures/Getty Images

Then-unknown actress Marilyn Monroe with Clifton Webb and Laurette Luez on the set of a 1948 comedy, "Sitting Pretty."

Marilyn Monroe, Clifton Webb, Laurette Luez, 1948

Loomis Dean/Life Pictures/Getty Images

Then-unknown actress Marilyn Monroe with Clifton Webb and Laurette Luez on the set of a 1948 comedy, "Sitting Pretty."

Marilyn Monroe, Clifton Webb, Laurette Luez, 1948

Loomis Dean/Life Pictures/Getty Images

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