Linus Roache has played upstanding characters on “Law and Order” and “Homeland” but he had to plumb the depths and reach the epicenter of narcissism to play Jeremiah Sand in “Mandy,” and it’s a frightening joy to behold. He talks about the draining yet rewarding time bringing this character to life and the guidance and trust he had from director Panos Cosmatos. Plus he answers a listener question about his character, King Ecbert, in the TV series “Vikings.”