Written and directed by "Whiplash" helmer Damien Chazelle, the project will mark 36-year-old Legend's first since 2008 film "Soul Man".
A contemporary love story about two dreamers who come close and then drift apart in quest of achieving their dreams, the movie also stars "Whiplash" actor J K Simmons, Finn Wittrock, Sonoya Mizuno, Jessica Rothe and Callie Hernandez.
Emma will essay the role of aspiring actress Mia who is lonely and desperate to fit in while Ryan will play jazz pianist Sebastian.