The film, which reunites Dano with Little Miss Sunshine helmers Jonathan Dayton and Valerie Faris, is written by Dano's real-life girlfriend Zoe Kazan, who also co-stars.
Dano plays a struggling writer who creates a character so vivid that she actually comes to life - that character being whimsical 26-year-old Ruby (Kazan).
Steve Coogan, Annette Bening, Elliot Gould, True Blood's Deborah Ann Woll and Arrested Development's Alia Shawkat also star.
Ruby Sparks is released in the US on July 25. A UK release date is yet to be confirmed.
Watch the trailer below: