I like Momoa, but honestly I don't picture him as Duncan Idaho. Admittedly, that's likely because in my mind when I read the novels my mind defaults to Edward Atterton, the actor from the Children of Dune miniseries.
Although, looking at the drawing of how Frank Herbert imagined the character, I suppose if Momoa cut his hair and shaved the beard, he could actually be a reasonable duplicate.
Although, looking at the drawing of how Frank Herbert imagined the character, I suppose if Momoa cut his hair and shaved the beard, he could actually be a reasonable duplicate.