The actress plays a commitment phobe who is forced to rethink her attitudes after her brother, played by Paul Rudd, is thrown out by his girlfriend and returns to shake up their sisters' lives.
Speaking to Ain't It Cool, Deschanel said: "I felt like it was really well cast. Like, I felt like [the cast] all looked like brothers and sisters, you know what I mean? It was cool, like, all of us brunettes with light eyes.
"The chemistry just happened."
My Idiot Brother co-stars Emily Mortimer, Steve Coogan, Elizabeth Banks and Rashida Jones.