Kevin Costner has sunk a mint of his own money into his projected four-part western epic Horizon; the first installment, Horizon: An American Saga — Chapter 1, is now making its rounds in cinemas, as Costner has always intended; he believes wholeheartedly in the big-screen experience. This first Horizon entry is a bit snoozy, albeit handsome-looking. Costner, who also co-wrote the script (with Jon Baird and Mark Kasdan), may not have the best control of this sprawling story peopled with a giant and somewhat confusing roster of characters. But his judgment is sound in at least one area: Sienna Miller plays Frances Kittredge, a pioneer wife who survives an Apache attack on the homestead she’s set up with her husband and two children. And if this first chapter of Horizon too often drags its boot-heels in the dust, it crackles to life every minute Miller is onscreen.

Miller has been working in movies and TV for 25 years now; pretty much ever…