Alexander Skarsgard says ‘The Northman’ was “physically and mentally the most difficult job I’ve ever had.” The 45-year-old actor plays Amleth in Robert Eggers’ Viking revenge saga and was literally “hackled and dragged through the mud” during filming. He said: “The days were really long and hard, and we were out in the mud and up on these mountaintops in the wind and the cold. “The week prior, I was working on the television show ‘Succession,’ on which I play a tech billionaire in a villa on Lake Como. So I literally went from playing one of the richest dudes on the planet in a crazy, beautif…