Honestly, Deadwood as a movie would probably remind me a lot of a movie called STRINGS. All the characters in it are marionette style puppets and the fact that they have strings attached to them is well known and incorporated into the lore of their world, but never breaks the fourth wall. They are a puppet show, but they don't know that they are.
I feel if Deadwood took a similar approach to this it would be really enjoyable. The whole of their world is just a show, but to them it is what really exists.
As far as style, I think going with a Jim Henson's Dark Crystal style of puppets would be the best choice. It would allow for the characters to have their cartoonish look about them, while still giving a realistic feel to the environment to feel like this world exists.
I feel like this movie would be best suited as an Adventure film with elements of horror and comedy. Just dark enough to be suspenseful, and just goofy enough to keep it from getting to dark.
Lathe- Jay Baruchel
Roguard- Vin Diesel (what? Don't look at me like that! He did a wonderful job as the Iron Giant! Yeah. That's what I thought )
Radley- Brian Mathis
???- Tim Curry (I don't know who he'd play, but he HAS to be in it!)
Plot: Not sure. Honestly would want Deadwood's developers to have a heavy influence on direction though. That way the story would stay true to their vision.
(Song - Band)
Stick Together - Eels
I'm Impressed - They Might Be Giants
Take Me Back - Kongos
Alexithymia - Anberlin
Stella - Cereus Bright (if there is a love interest in movie)
This is just my spur of the moment thoughts on this, so my ideas might change, but I think a movie based around this would be killer!