Clive Standen originally auditioned for the part of Ragnar, but actually wanted Rollo. Travis Fimmel then came along and was cast as Ragnar and Clive Standen got to be Rollo instead. So in the end, everybody got what they wanted.
The show was originally about Alfred the Great fighting the Great Heathen Army.
Ragnar was supposed to die in season 1. Thank God they didn't do that.