Paul Dano stars as Calvin, an isolated young author who has been suffering from writer's block ever since the release of his acclaimed first novel.
When he writes the character of Ruby, a free-spirited young woman with just enough personality quirks to be charming, he feels such a connection to her that he wills her into existence.
Speaking to Digital Spy, Kazan explained that she hadn't originally set out to write the two lead roles for herself and real-life boyfriend Dano, and that he brought up the possibility while reading an early draft of her script.
"He asked, 'Are you writing this for us?' I obviously was in some subconscious way," she admitted.
Kazan went on to discuss the "manic pixie dream girl" archetype. The term was coined by film critic Nathan Rabin to describe characters like Kirsten Dunst's in Elizabethtown and Natalie Portman's in Garden State - bubbly, girlish kooks who exist largely as an accessory to the male protagonist's emotional arc.
"I wasn't consciously trying to [comment on that]," Kazan mused. "I think probably my perspective on that kind of character comes through in the movie.
"But it really wasn't until people started saying it to me, like, 'Look what you've done', that I noticed it. The larger picture is far more important to me - we all put a box around each other when we meet.
"People are so much more complicated than any box you could put on them, including a 'manic pixie dream girl' box."
> Paul Dano talks 'Ruby Sparks', reuniting with 'Sunshine' directors - video
> 'Ruby Sparks' review: Locarno Film Festival 2012
Ruby Sparks is released in the UK on October 12. Watch a trailer below: