Most film productions go through a casting period that can take weeks, if not months, as the producers make sure they hire the right talent for the job by having multiple actors try out for the roles.
This is a pretty normal part of the movie making process, but some filmmakers don't want to deal with the whole casting call at all- because they already know exactly who they want to play the role.
As you'll see in this video by Looper the Hellboy movies were written with Ron Perlman in mind for the lead, and the role of Drax the Destroyer in Guardians Of The Galaxy was only offered to Dave Bautista.
But John Carpenter's casting of Rowdy Roddy Piper in They Live seemed strange to some at the time, and now we can't imagine anyone else kicking ass and chewing bubblegum quite like Rowdy Roddy!
-Via Laughing Squid