According to Variety, Beach (Flags of Our Fathers, Windtalkers) will play Nat Colorado, a half-Apache who works for Col. Woodrow Dolarhyde (played by Harrison Ford).
If you don't know the plot(although I think the title is pretty self explanatory!), here's the official synopsis..
1873. Arizona Territory. A stranger (Craig) with no memory of his past stumbles into the hard desert town of Absolution. The only hint to his history is a mysterious shackle that encircles one wrist. What he discovers is that the people of Absolution don’t welcome strangers, and nobody makes a move on its streets unless ordered to do so by the iron-fisted Colonel Dolarhyde (Ford). It’s a town that lives in fear. But Absolution is about to experience fear it can scarcely comprehend as the desolate city is attacked by marauders from the sky. Screaming down with breathtaking velocity and blinding lights to abduct the helpless one by one, these monsters challenge everything the residents have ever known Now, the stranger they rejected is their only hope for salvation. As this gunslinger slowly starts to remember who he is and where he’s been, he realizes he holds a secret that could give the town a fighting chance against the alien force. With the help of the elusive traveler Ella (Olivia Wilde), he pulls together a posse comprised of former opponents—townsfolk, Dolarhyde and his boys, outlaws and Apache warriors—all in danger of annihilation. United against a common enemy, they will prepare for an epic showdown for survival.
I still haven't read the comic, and don't intend to to be honest! The movie looks like a lot of fun though. And check out the cast. Aside from Craig, Ford, Wilde and now Beach, we have Sam Rockwell, Paul Dano, Clancy Brown, and Keith Carradine. So far all we have seen of the actual movie is a test shot featuring Daniel Craig as the cowboy stranger. The screenplay was written by Star Trek scribes Roberto Orci and Alex Kurtzman, alongside Lost co-creator Damon Lindelof. Cowboys and Aliens is expected to open on July 29th 2011.