I actually made the following fantasy cast list last night:
Roland

Dennis Quaid, so long as you could keep him from grinning like an idiot like he seems prone to do. Has dark hair, blue eyes, and looks especially rugged when he's sporting a 5 o'clock shadow. Plus he has the deep voice needed that other possibles like Hugh Laurie and Viggo Mortensen sort of lack.
Eddie

Jake Gyllenghaal. The pic is just an example of him with shaggy hair, though he'd need to wear hazel green contacts.
Susannah

Jada Pinkett looks the part to me and I think she could pull off a Detta pretty well.
Jake
I think Jake's actor doesn't need to be famous. Any young teenage boy with sandy blond hair and blue eyes could fill the role.
I also had some ideas for some of the more minor characters.
Walter
I can't think of anyone who could possibly be better than Willem Dafoe. As Randall Flagg though they might want to have a different actor.
Callahan
James Cromwell. What can I say? They did a good job casting him in 'Salem's Lot.
Mia

Claire Forlani. She has the face, hair, body. I think she could pull off the "innocent" look better than someone like Angelina Jolie for instance.
Eldred Jonas

Sam Elliott. Nuff Said.
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