I am a HUGE fan of the series, that being said I'm only casting the primary Ka-tet and villain, the majority of the other characters would be very small parts until later films. I'm also not including Callahan (sorry) as he wouldn't appear until Wolves of the Calla.
Roland Deschain - Viggo Mortensen
The man can act. Putting aside the fact that he looks the part and has experience in this kind of role, he seems to be cast in a lot of heroic and/or wise-to-a-fault roles
Jake Chambers - Joel Courtney
After watching Super 8, I'm convinced he was made for this role.
Eddie Dean - Jared Leto
While the possibility of this happening are slim, Jared has the experience of playing a heroin addict in Requiem for a Dream. Also, if Chris Nolan has taught us anything its that putting actors out of their element is good for them and generally for their roles, and having Jared in this role could potentially be as pleasantly surprising as Gary Oldman's Gordon, or Ledger's Joker
Susannah Dean - Zoe Saldana
Uhura... Is that not enough to prove that she's not only leading actress material, but hero material?
Walter O'dim - Kevin Spacey or Gary Oldman
This was incredibly difficult to cast as this would be an incredibly difficult role, and while Kevin Spacey is my first choice (primarily for his work in Se7en), Oldman is a seasoned villain, and can pull off the creepy mystery of the character
Oy - practical effect or animatronic
If I see an unconvincing CGI Oy, I will kill everything.
As a bonus I'll leave you with my pick for Eldred Jonas, I know I said I wouldn't cast anyone else, but this idea was just too damn good
Eldred Jonas - Michael Wincott
Watch The Crow. That's all I have to say on the subject