He joins Josh Brolin, George Clooney, Channing Tatum, Jonah Hill and Scarlett Johansson in the pic.
Universal will distribute with the Coens producing, along with Eric Fellner and Tim Bevan for Working Title Films.
The story follows a Hollywood fixer in the 1950s who is working to keep a studio’s stars in line.
“Hail Ceasar!” looks to follow in the steps of so many other films from the siblings where actors and actresses are doing whatever they can to be involved in the project some way.
Ehrenreich was last seen in “Blue Jasmine” and recently wrapped production on the untitled Warren Beatty project.
He is repped by CAA and Brillstein Entertainment.