It is rare to find cowboys on the silver screen who spend much time performing the humdrum labor—herding cattle—that gave their profession its name. Westerns suggest that cowboys are gun-toting men on horseback, riding tall in the saddle, unencumbered by civilization, and, in Teddy Roosevelt’s words, embodying the “hardy and self-reliant” type who possessed the “manly qualities that are invaluable to a nation.”

But real cowboys—who worked long cattle drives in lonely places like Texas—mostly led lives of numbing tedium, usually …