Sienna Miller and Sam Worthington join Kevin Costner's 'Horizon.'

Sienna Miller and Sam Worthington join Kevin Costner's 'Horizon.' ...

Sienna Miller and Sam Worthington have signed on to star in Horizon, following previously announced cast members Kevin Costner and Jamie Campbell Bower. The film, which is being filmed later this month, is a personal passion for Costner, who is expected to return to the director's seat for the first time since 2003.

Horizon is said to be the first film in a planned trilogy to cover a fifteen-year period that will encompass pre- and post-Civil War expansion and settlement of the American West. Through a variety of perspectives, the story will investigate interactions with the Native peoples native to the American West before colonizers' attempts to settle the land and claim it for themselves.

Miller is an actress who is well-known for her roles in such films as American Sniper, Foxcatcher, and Layer Cake. Worthington is best known for his roles in Avatar, which has yet to be released internationally since the film ten years ago, and will return for a sequel later this year.

Worthington said, "I am extremely excited and humbled to embark on this incredible and significant journey with Kevin," who is one of the industry's finest and most inspiring storytellers. The narrative is moving. I am very proud to be a part of it."

Costner is an Academy Award-winning filmmaker best known for his film Horizon, which also won the best picture. Costner wrote the script for Horizon with Jon Baird, and is now financing the film.

Horizon will begin filming in Utah at the end of the month. Keep an eye on Collider for any updates on the project.