Man so many different ideas for Roland, although if you look at them close enough they all sort of look the same. So the casting director has a lot of viable options to work with, it will all depend on the actors themselves. Whether or not they agree to the project, whether or not they are even available for shooting (if they are filming different projects), and all other aspects of business. Say everyone collectively decides they want Viggo to be Roland, but Viggo makes a public statement saying he has no interest in being Roland, then they move along down the line of casting subjects. Say they offer it to Di'Caprio, but cannot offer him enough money for the role OR he's too busy filming other films to participate, again they must run through the list.
Which still makes Mount a viable option because he's currently available, would not request an inordinately large paycheck, and would probably express interest in the role if given the opportunity. There is a lot more that goes into making a casting decision than capability, compatibility, and likability. So many factors can affect the casting of a role, especially in a movie so up in the air as Dark Tower. A film that has been intended to be a film for years now, has run down the list of directors over and over again before settling on one. What if an actor dislikes the director and refuses the role because they do not want to work with them? Or what if they look at the script and their agent decides it's not suitable for their client and encourages the actor to pass?
Etc, etc, etc. I do like all the suggestions made, and I'm actually working on a fan-made teaser trailer for the movie with some of the casting suggestions on this list. It'd be interesting to have it wrapped up nice and tight and showed to the casting director to encourage them not to make a huge casting mistake. Which looking at their choice for Roland right now, universally the Dark Tower fanbase is in agreement that would be a casting mistake. It just does not work the way they see it in their minds, I can understand why they might have thought that way, but Idris Elba is simply no Roland.
Anson Mount, Viggo, Di'Caprio, sheesh ANYONE OF THESE MEN would be suitable. All suggestions in this thread work so much better, probably because everyone here has such a love for the books (at least I do) and has come to know each and every character in the series personally. If you feel like you know Roland, and you see him in your head, then see them picking a person who is the polar opposite it incites backlash. A studio does not want backlash because backlash means low box office, so I'm hoping they'll see these things through logically and pick a cast that will encourage fans to see the film rather than drive them away from it.