This news comes from director Scott Coffey, who tweeted at The Playlist to say, "thrilled that Dan Boeckner of Wolf Parade & Handsome Furs is scoring my movie #adultworld starting @RobertsEmma & @johncusack."
That's all the news we have regarding Boeckner's involvement, so there's no information as to whether he will be writing full-blown rock songs with vocals or if these are cinematic mood pieces.
According to IMDB, the Cusack-starring Adult World is about an aspiring poet who ends up working at an adult book store. It's tentatively due out this year.
In related news, Handsome Furs cancelled their recent European tour due to what their website calls an "urgent health issue."