Who hasn’t entertained a fantasy of romantic omnipotence in which a dream partner complies with your every wish? In the language of“My Fair Lady,” “Oh, wouldn’t it be loverly?”

But wait. Would unquestioning obedience really answer your deepest longings? Or would it rob you of the thrill of the chase? Might playing puppet master awaken inner demons that drive you to behave monstrously?

“Ruby Sparks,” an ingenious and delightful variation on the Pygmalion myth, toys whimsically with these questions, as the initially perfect relationship between a neurotic author and his sprung-to-life fictional creation goes through changes.

The movie’s protagonist, Calvin Weir-Fields (Paul Dano), a shy, self-absorbed Los Angeles writer, is a literary whiz kid. His first novel, which he wrote at 19, was a best seller; a decade later, its reputation ranks just below that of “The Catcher in the Rye.” Calvin’s problem is how to follow it up.

While struggling with his writer’s block, he meets his Galatea, Ruby Sparks (Zoe Kazan), during a walk in the park with his dog, Scotty (named after F. Scott Fitzgerald). This smart, saucy redhead, first glimpsed as a hazy silhouette emerging out of the sunlight in a brief opening dream sequence, is a figment of his imagination. But once literary inspiration seizes him, she takes shape as he invents her character on his typewriter.

He awakens one morning to find Ruby, who by now has a back story, downstairs making breakfast. She’s a 26-year-old painter from Dayton, Ohio, who has moved around the country a lot. Her idols are Humphrey Bogart and John Lennon.

The only blight on Calvin’s happiness is his fear that he has gone crazy, and he tries fruitlessly to wish her out of existence. It gets weirder when his worried brother, Harry (Chris Messina), finally meets Ruby and confirms that she is not a hallucination. Harry suggests that Calvin test out the theory that she is a product of his imagination by fleshing out her character with more quirks and determining whether she changes under his dictation.

“Ruby Sparks,” directed by Jonathan Dayton and Valerie Faris, the team behind “Little Miss Sunshine,” has the same zany and sweet tone laced with just enough hardheaded wisdom to keep it grounded in psychological truth. The screenplay, by Ms. Kazan, is so polished and witty that it immediately puts her in the same league as Diablo Cody. And Ms. Kazan’s lovely, tart performance is the equal of Ellen Page’s portrayal of the title character in “Juno.” Both are impetuous screwball heroines who could have been created only by women.

I shudder to imagine how the same concept might have been treated by the writers in the Judd Apatow school of eternal adolescence. There would be smirking, misogynist jokes about sexual organs, bodily fluids and performance anxiety, and the poor woman would be subjected to humiliating trials.

But Ms. Kazan’s screenplay, which treats Ruby and Calvin (the lead actors are a real-life couple) with equal respect, refuses to go there. What a relief to see a funny romantic comedy whose humor stays above the waist.

Mr. Dano’s Calvin, with his combination of sheepish nerdiness and intellectual arrogance, suggests a softer-edged, better-looking Woody Allen character. The first time he realizes the degree of his control over Ruby is when he writes that she speaks fluent French. Moments later that is all she speaks.

The movie goes on a madcap tangent when Calvin’s mother, Gertrude (a twinkling Annette Bening), persuades him to bring Ruby to her house in Big Sur. Gertrude and her partner, Mort (an amusing but miscast Antonio Banderas), who designs avant-garde furniture, are superannuated hippies so radiant that they make Jane Fonda’s bohemian grandma in “Peace, Love & Misunderstanding” seem dour.

As Calvin’s manipulative experiments continue, Ruby responds with alarming intensity. One moment she is clinging and dependent, and the next she is maniacally buoyant. When Calvin decides to grant Ruby some independence, and his lecherous editorial mentor (Steve Coogan) puts the moves on her at a literary party, he can’t handle it.

The games reach a hysterical peak when Ruby faces Calvin as he sits at his desk, frantically typing instructions, which she tries to resist but can’t. The more conflicted their relationship, the further the movie ventures into Charlie Kaufman-Michel Gondry territory without twisting itself into intellectual knots.

“Ruby Sparks” doesn’t try to pretend to be more than it is: a sleek, beautifully written and acted romantic comedy that glides down to earth in a gently satisfying soft landing.

“Ruby Sparks” is rated R (Under 17 requires accompanying parent or adult guardian). It has sexual references, some strong language and drug use.

Ruby Sparks

Opens on Wednesday in New York, Los Angeles, Washington, San Francisco and Chicago.

Directed by Jonathan Dayton and Valerie Faris; written by Zoe Kazan; director of photography, Matthew Libatique; edited by Pamela Martin; music by Nick Urata; production design by Judy Becker; costumes by Nancy Steiner; produced by Albert Berger and Ron Yerxa; released by Fox Searchlight Pictures. Running time: 1 hour 44 minutes.

WITH: Paul Dano (Calvin Weir-Fields), Zoe Kazan (Ruby Sparks), Antonio Banderas (Mort), Annette Bening (Gertrude), Steve Coogan (Langdon Tharp), Elliott Gould (Dr. Rosenthal), Chris Messina (Harry), Alia Shawkat (Mabel), Aasif Mandvi (Cyrus Modi), Toni Trucks (Susie) and Deborah Ann Woll (Lila).