There are documentaries, and there are comedies made to look like documentaries, and Paper Heart is both. Conceived by comedian Charlyne Yi and filmmaker Nicholas Jasenovec, it combines elements of reality and fiction in an amusing, meta-referential way, though one's enjoyment of it may ultimately come down to one's enjoyment of Yi as a performer.
It is set up as a documentary about Charlyne's search to determine whether true love really exists. She doesn't think it does -- or, at the very least, she thinks she's not capable of feeling it. (I can't imagine anything sadder than being unable to experience romantic love, but that's beside the point.) To investigate, Charlyne travels the country to interview biologists, old married couples, and Las Vegas wedding chapel officiators who dress as Elvis. Those segments are real, like you'd find in any documentary.
But in the process of making the documentary, Charlyne meets actor Michael Cera at a party, and they start tentatively dating. The documentary director (played by actor Jake Johnson), knowing a good thing when he sees one, insists on following Charlyne and Michael around. It's a no-brainer, really: She's making a movie about love, and in the meantime starts dating someone? Perfect!
Those segments are loosely scripted, of course; while Michael Cera and Charlyne Yi have been romantically linked, the film is not a real chronicle of their relationship. But the film plays it straight, shooting all the scenes, real and non-real, in the same way, as if they were part of the same documentary. Savvy viewers are meant to understand where the line is, but I wouldn't be surprised if some audience members come away thinking the whole thing was a straightforward documentary. A lot of people thought The Blair Witch Project was real, too.