IMDB bills this as "Comedy/Fantasy/Romance", the same small genre intersection as (say) Midnight in Paris. But I liked this a lot better.
Paul Dano plays Calvin Weir-Fields, a writer whose first novel was a critical and commercial success, allowing him to live in a shiny house with a great view and a pool. But he's struggling and failing to put words to paper, and his life is lonely emptiness, enough to see a sympathetic shrink (Elliot Gould).
Calvin is obsessed with a (literal) dream girl, Ruby. The shrink gives Calvin his assignment: write about her. And—oops—this is sufficient to make Ruby appear in real life. (At first, Calvin takes Ruby for a delusion, and doubts that anyone else can see her. This is hilariously debunked.)
All is bliss for a while. But things go wrong in an unexpected way: Ruby is (again, literally) an unfinished character. Her reality is limited by what Calvin has typed, and her incompleteness begins to chafe.
Annette Bening and Antonio Banderas are very funny playing Calvin's mom and stepdad.
One impressive bit: this seems to be a male fantasy, "obviously" written by a guy. But no: the writer is Zoe Kazan, who also plays Ruby. (It's in her genes: grandpa was Elia Kazan.)