Steve Martin has never been shy about his intellectual aspirations. But he has seldom taken them seriously. “‘The Apple Pie Hubbub’ was a significant novel for me,” Mr. Martin says in “A Wild and Crazy Guy,” the platinum-selling recording of his early stand-up, “because that’s when I first started using verbs. My novels really brightened up after that.” Now that Steve Martin really does write novels, it hardly seems right to take them seriously, either.

Yet “Shopgirl,” his debut novella of 2000, now a movie, was not very funny. It had a sweet, pastel modesty, largely derived from Mirabelle, its eponymous shopgirl. Mr. Martin’s second novella, “The Pleasure of My Company,” was less successful because it had a paranoid man as a narrator, a man Mr. Martin has played out in his movies. “Shopgirl” put Mr. Martin in the body of a serious young girl from Vermont. Its author came off not as Steve Martin, the celebrity novelist, but as Sensitive-Paternal Guy, who wrote a book that is elegant and restrained.

The movie version of “Shopgirl” fritters away much of that modesty. Mr. Martin is not only screenwriter (that is to say, adapter), but the producer, the narrator, and the male lead. As an artistic maneuver, it’s a blitzkrieg. But Mr. Martin is coy. “I originally thought there was no way this novella could ever be a movie because it was so interior,” he says in the press notes. “But then, a year after it was published, scenes just started popping into my head.”

How did Steve Martin go from the epochal idiocy of his early days, from “Excuuuuuuuuuse Me,” to the creation of a movie like “Shopgirl,” a film of lush, unstinting sobriety? When did Steve Martin decide to become an auteur? Did the idea just pop into his head?

In his introduction to his collected essays, “Pure Drivel,” Mr. Martin again claims inadvertency. He swore off movies for years, he explains: “During these years, in which I vowed to do nothing and leave myself alone about it, I accidentally produced several plays, a handful of sketches, two screenplays, and a reorganization of my entire self.” Lucky guy.

When Mr. Martin first began to write for the New Yorker, in 1996,Tina Brown was increasing that magazine’s coverage of fashion and popular culture. Mr. Martin, then and still, was a kind of magic bullet: the most famous person in the world who wanted to be less famous and more intellectual. In short, he was the exact converse of Tina Brown’s New Yorker; they met halfway.

The artistic compulsion is figured in a similarly casual manner in Mr. Martin’s ars poetica, “Writing is Easy!” Sweating is not to be admitted, even as a joke. According to the secretly cynical speaker, “Writer’s block is a fancy term made up by whiners so they can have an excuse to drink alcohol. Sure a writer can get stuck for a while, but when that happens to real authors, they simply go out and get an ‘as told to.’ The alternative is to hire yourself out as an ‘as heard from,’ thus taking all the credit.”

Like most Shouts & Murmurs pieces, Mr. Martin takes a voice, gives it a ridiculous goal, and lets it betray itself. But Shouts & Murmurs was no procrustean bed for Mr. Martin. However pretentious or even calculating his contributions to the New Yorker appear, they did not significantly change his original formula. He made his name as a stand-up by making fun of stand-up itself, just as his magazine pieces make fun of writing.

“I think there’s nothing better than for a person to come out and do the same thing over and over for two weeks,” he said at the end of one two-week gig. “I’m going to do the same joke over and over in the same show; this will be like a new thing.” And then he flubs his lips: nuh nuh nuh, idiotically. The premise was always to be smart and idiotic. Yes, he would be smarter than most comics. But he would be dumber, too.

Perhaps Mr. Martin’s trajectory, from comedy clubs to “The Jerk” to “Parenthood” to the New Yorker, really was a series of sudden light bulb thought-bubbles, perhaps it was a calculated career arc, but one thing is certain. The idea that Mr. Martin’s more serious artistic productions are accidental is entirely consonant with the original premise of his humor: that artistic production itself is deeply silly.

While his novels seem to consolidate the trend of respectability in Mr. Martin’s career, in fact they represent something new, something alien to the rest of his career. “Shopgirl” represents a departure from Mr. Martin’s founding premise. The only conclusion is that Mr. Martin has finally made the artistic stab – the hilarious scimitar swipe – that he so fruitfully lampooned.

As a sincere expression, “Shopgirl” shows the didactic side of Mr. Martin’s personality. If Mr. Martin’s mid-career blockbusters, “Parenthood,” “Dirty Rotten Scoundrels,” and others, profited from the frisson of a doofus in positions of power, “Shopgirl” turns the doofus into a quiet wizard. “What Mirabelle needs is some omniscient voice to illuminate and spotlight her,” says the voice-over, and that omniscient voice is Steve Martin. September-May relationships are seldom so paternalistic. At the film’s end, you will feel that this man, Steve Martin-cum-Ray Porter, has put Mirabelle through emotional boot camp.

Why has Mr. Martin gone straight? Because he has always wanted to. Why did it take him so long? Because he’s much better as a comic.

My So-Called Girlfriend

Watching “Shopgirl,” the movie, means evaluating Mr. Martin as an actor among other actors. Jason Schwartzman, in the role of Mirabelle’s other, more appropriate suitor, is a tedious slacker, his neuroses glibly conceived. He now has none of the novelty he had in “Rushmore.” Claire Danes is an interesting shopgirl; she can look haggard one minute and lovely the next, but, as in “My So-Called Life,” she is at her best when narcissistically resolute, as after a teenage crisis, soldiering on in disbelief.

Mr. Martin plays Ray Porter, a Seattle-based logician, afraid of intimacy, and rich. He is the straightest of all straight men. Watching Mr. Martin in this role is fascinating. When Ray betrays Mirabelle, Ms. Danes collapses, like a child, but Mr. Martin is placid, intent, and empathetic but not himself upset. When he takes Mirabelle to refill her antidepressant prescription, he is like nothing so much as a village elder in an icy utopia.

As a film, “Shopgirl” lands just outside the developing genre of quirky hipster dramas. Second only to Mr. Martin’s performance, for example, is the film’s color palette: In many ways, “Shopgirl” is like “Lost in Translation,” except with a pulsing orchestral soundtrack. Mr. Martin, in his paternalism, explicitly spells out Ray and Mirabelle’s problems; his script would have benefited from some fashionable ellipses. “Shopgirl” is easy on the eye, and on the heart, and on the brain.

The narrator’s closing speech – “He had hurt them both, and he cannot justify his actions except that, well, it was life” – gives the audience the impression that Mr. Martin’s longtime interest in philosophy is not analytical so much as aurelian, and that artsiness, as opposed to comedy, is all about quiet consolations.