Rockwell is joining Daniel Craig, Olivia Wilde and Harrison Ford in the film, based on the Platinum Studios graphic novel. The sci-fi Western explores what would happen if the traditional Old West enemies — cowboys and Native Americans — found the prairie attacked by aliens in mid-1800s Arizona.
Rockwell will play a bar owner named Doc who joins in the pursuit of the aliens.
Favreau will start shooting the film in June. Alex Kurtzman, Roberto Orci and Damon Lindelof wrote the script.