Well Viggo isn't really tall or dark enough to match the book description for me but he had such a strong physicality and was similar enough to Bean and Wenham to create the illusion of a shared heritage that I think the casting worked.
However, this is the online version of an interview I read in the paper the other day which may be of relevance to this thread.
http://www.telegraph.co.uk/arts/main...fmorten126.xml