Having just watched "The F Word" recently, which also stars Zoe Kazan, I said of her: "... what a legacy she has: her grandfather was Elia Kazan, one of Hollywood's most famous directors, both her parents are well-established screenwriters, and she herself is also a screenwriter and playwright." This is where she shows what she's got as a writer: she wrote both the story and the screenplay.
Paul Dano plays Calvin Weir-Fields, a young and very successful novelist who's not very good at relationships. He's having trouble working on his next novel, and at the behest of his therapist starts writing about a young woman. Things related to her start appearing in the real world, and eventually she herself appears (the titular Ruby, played by Kazan). Having established that other people can see her, he fully accepts her into his life and is ecstatically happy. Except of course that it doesn't all go according to his not particularly realistic plan.
The fallout fits fairly well with Ruby's origins, although the central conceit is never explained. But the movie struck me as being clever (witty, mildly interesting) without actually being intelligent (it never got much beyond its one big idea), and so it's kind of a one shot, not worth a second viewing. I also didn't feel that Calvin deserved even as positive an ending as he was given, but then moral worthiness and positive outcomes are only strongly linked in children's books. Kazan is worth watching as a writer, but give her a few more years.