A curious case, this “Benjamin Button.” Far afield from his sublime novels, such as “The Great Gatsby” and “Tender Is the Night,” or his finest short stories, from “The Diamond as Big as the Ritz” and “Winter Dreams” to “The Last of the Belles,” F. Scott Fitzgerald’s “The Curious Case of Benjamin Button” is a doodle — a snarky little tale about a man born old who ages backward — that Fitzgerald whipped out, probably mainly for the cash.

To build a Movie as Big as the Ritz out of such a trifle is only part of the reason why the development of “Benjamin Button” consumed two decades and involved at least two screenwriters’ best efforts, more than a few directors and the patience of producers Kathleen Kennedy and Frank Marshall, as well as former Paramount topper Sherry Lansing, for whom this might represent her last major legacy to the studio she oversaw.

But this being Fitzgerald, a true American treasure, screenwriter Eric Roth says that “it’s a tightrope talking about the merits or demerits of his original story, because I don’t want to be disrespectful to an artist who’s 100 times the writer that I’ll ever be.”

Once Roth was onboard with the long-

gestating project and consulted with Fitzgerald scholars such as A. Scott Berg, he learned that, in fact, the tale was “a whimsy … of no consequence, but that isn’t to impugn Fitzgerald’s motives.

“Underneath this surface of a cynical, goofy yarn, there’s a feeling of time lost, of a doomed life, that runs through so much of his major fiction,” Roth says. “When I first read it, I wasn’t sure what I could do with it, because it didn’t seem solid enough. But as I spoke more with Kathleen and Sherry, things changed, my perceptions changed.”

Kennedy reflects that “from the start, when we first read the story in the mid-’80s when Frank and I were with Steven (Spielberg) at Amblin, the primary premise was the idea of a man and a woman meeting in the middle of the story, sharing some time and love together, and then time’s passage taking it away. Eric was able to make that emotionally resonant.”

Since the original story treats Benjamin’s love life briefly in passing, and without any of the heartbreaking sense of romantic loss that lies at the heart of the best Fitzgerald, this key premise had to be invented for the film — a surprising fact for those viewers familiar with the author’s work but unacquainted with the story. “With a work this obscure,” Roth notes, “you have far more liberties, since a fanbase isn’t looking over your shoulder. So nobody was likely to object when the object of Benjamin’s affections, Hildegarde, was turned into Daisy for the film.”

Daisy, incidentally, was also the name of Benjamin’s love in the script version previous to Roth’s involvement, written by Robin Swicord. Roth is struck by this coincidence — “Robin and I were both clearly thinking about ‘Gatsby,’ and his lost love, Daisy” — though Kennedy wonders if perhaps the idea was floating around during the endless hours of discussions across the years.

“When you work on something this long, and you have the shadow of Fitzgerald in the room, which we talked about constantly,” says Kennedy, “various people get various ideas, and they evolve over time and discussion, and you can actually lose track of who had what idea.”

What is even more fascinating than the presence of a character named Daisy in “Button” is how the movie’s Benjamin makes a choice that’s utterly different from the story’s Benjamin. While Hildegarde, who meets Benjamin when he’s in his prime, stays with him for years, gives birth to a son and grows old and bitter as Benjamin grows younger, the movie Benjamin makes the deliberate decision to tell Daisy to leave him even while they’re still deeply in love, knowing that a future with him is impossible.

“It’s almost as if our Benjamin had read Fitzgerald’s story and received a warning,” says Roth. “This was also a way of giving tribute to Fitzgerald. Certain themes keep popping up in writers’ work, whether it’s good or bad, and many of my scripts deal with doomed lovers, the passage of time, the temporality of life.”

The choice to leave Daisy also makes Benjamin a bit more active. Kennedy acknowledges that “the universal, classic note that we received from everybody who read the script was, ‘He’s so passive.’ I think more than any other single thing, even the technical challenges of visualizing his reverse aging process, this sense of his passivity was the biggest roadblock for the movie getting made. But the reality remains that this is a central character who’s the observer of his own life as it passes him by.”

For Roth, getting past the medical questions of the odd little Benjamin — an issue that Fitzgerald simply leaped over in the grand American style of the tall tale — was a real problem to be cracked. Solution: Create the fable of the clock that tick-tocks backward, a device just fantastic enough to posit a cause for Benjamin’s freakish condition.

“I think most of all, I was writing this as my mother was in the course of dying,” Roth recalls, “and this just made everything so much less abstract, and far more personal. It would seem impossible to get inside this tale in a directly personal way, but that’s what happened, and it let me insert more emotion than I had expected.”