Matthew McConaughey and Idris Elba have confirmed (via Twitter) that they will be playing the role of the Man in Black and Roland Deschain in Sony's movie version of The Dark Tower. Stephen King has also confirmed the news, also reporting that the first line of the script is the same as in the books: "The Man in Black fled across the desert and the gunslinger followed."
However, to the consternation of some fans, the first film will not adapt The Gunslinger, the first novel in the series. Instead, it will be mostly set on contemporary Earth. That also rules out the next likely starting point, Roland's backstory detailed in the fourth novel in the series, Wizard and Glass. The setting of the story in the "real" world rather than Mid-World is a curious one, and may suggest they are taking the story in a different direction to the novels.
Further details should be made clear in the next few weeks as production gets underway (the film starts shooting in May). However, King does suggest in the linked interview that the reason for the film starting on Earth is that the earlier part of the story on Mid-World would make for a better TV series (possibly with a younger actor playing Roland) as a prequel spin-off rather than in integral part of the narrative, as Ron Howard had been planning.