My first choices (with character description):
Roland Deschain - Last of the Gunslingers, a knightly order in a world that has 'moved on.' Agewise, he should be hard to pinpoint. As the series is somewhat of a Western mixed with Fantasy elements, he should have some grittiness to him. My first choice: Viggo Mortensen Alternate: Daniel Craig
Eddie Dean - A heroin addict who joins Roland on his quest for The Dark Tower. Eddie should be in his early 20s and have the look of being addicted for several years. My first choice: Nicholas Hoult Alternate: Daniel Radcliffe
Susannah Dean - A wheelchair-bound black woman who lost both her legs when pushed in front of a train. Has a split personality to start with. Should be in her mid-to-late 20s (if I am remembering correctly). My first choice: Lupita Nyong'o Alternate: Shareeka Epps
Jake Chambers - A young boy of 11 whom Roland first encounters while pursuing the Man in Black, and later reunites with due to a paradox. No choices for this one, as casting this role will be exceptionally hard since Jake shouldn't age much throughout the 7 books, which will be tricky for the producers. I could see them aging him slightly to lessen that problem.
The Man in Black aka Randall Flagg - A sorcerer being pursued by Roland. Should be another that appears ageless, and should also be able to be both charming and dangerous at the same time. My first choice: Michael Fassbender Alternate: Daniel Dae Kim
Jack Mort - A sociopathic man who randomly pushes people (he is the one that kills Jake originally, and is responsible for Susannah losing her legs) and drops objects onto them to kill them. No age given, so that can be played with. My first choice: Oscar Isaac Alternate: Jonah Hill
There are other prominent roles, but those are the biggies for the first two books. Who would you cast?