Well, I did say cast the right person for the role.
At the time, of John Wayne playing Genghis Khan, it didn't matter what the movie was about... it was a John Wayne movie. They just grabbed the next script off the pile, and put Marion in the starring role.
In that case, if you wanted a John Wayne movie, then he was the "right person for the role".
If you wanted a Genghis Khan movie, then he was the wrong person.
Again, Linda Hunt.