I walked into "Paper Heart" knowing nothing about it and walked out feeling both charmed and conflicted. The picture is presented as a low-budget documentary about a low-budget documentary being made by Charlyne Yi (the cute stoned girlfriend in "Knocked Up"). The subject is love: Yi doesn't believe in it, but she wants to find out why so many people do. So she and her producer, Nicholas Jasenovec, and cameraman Jay Hunter set off around the country to learn more.

As they traipse from Amarillo to Albuquerque, and Atlanta to Palm Springs to New York City, we watch Yi bringing her microphone to bear on all sorts of everyday people. An old woman tells her love is a lightning bolt. A college professor says it's a molecular event. She talks to bikers and psychics and high-school sweethearts, and gets a funny scene out of an encounter with an Elvis impersonator in a Las Vegas wedding chapel. To vary the talking-heads texture of the movie, Yi illustrates some of the stories being told to her with elaborate home-made puppet tableaux — which sounds dire, but is actually kind of sweet.

The film's key moment takes place at a party in L.A., where Yi meets Michael Cera, who, like herself (and Seth Rogen, who also puts in an appearance), is a veteran of the Judd Apatow laugh factory. Cera is intrigued by her project. He's intrigued by Yi. Soon they're IM'ing each other, then having lunch dates, going bowling, playing Frisbee, shopping for musical instruments. (Along with being an actress and a standup comic, Yi is also a songwriter.) Jasenovic is filming all of this — Yi has agreed that he can shoot everything — and when he sees his star and Cera walking along a beach hand in hand, he calls out a request: Would they maybe like to kiss right about here? It'd be perfect. Even from a distance, we can see that Cera is annoyed: "No," he says, "this is fine."

As eventually became clear, even to a dullard like myself, "Paper Heart" is not a documentary. It's a scripted film (written by Yi and Jasenovic) with what I think we can assume is a certain amount of improvised dialogue. How much improvisation is a niggling concern — were the interview subjects coached to be funnier, more touching? Yi and Cera apparently actually have, or had, some sort of romantic relationship; and Jasenovic, while he is the director, is played on-camera by an actor, Jake M. Johnson. The result of such narrative calculation is to sabotage the film's initially beguiling effect: What seems at first like a fresh new kind of documentary is really just a familiar geek-love exercise in meta-media drag.

Still, the picture has a light, winsome appeal. Yi, with her bright eyes and scrunchy grin, has an elfin charisma that wears well (for the 88 minutes that the film runs, anyway). And Cera, underplaying his (or his character's) mounting resentment about the camera's intrusion into a personal area he considers off-limits, allows the movie, in a small way, to get at something important — the endangered state of private affinities in the age of rampant "reality" documentation. This isn't the payoff I expected at the movie's outset, but then what could I possibly have been thinking?

Check out everything we've got on "Paper Heart."

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Don't miss Kurt Loder's reviews of "Julie & Julia" and "Cold Souls," also new in theaters this week.