Starting in 2001, movie fans from around the globe came to know and love Viggo Mortensen as the noble, charismatic leader Aragorn in “The Lord of the Rings” trilogy. But for his last two major film projects, his characters have teetered in the middle before traveling to the opposite end of the heroic spectrum, proving that he’s not so concerned with his screen image as much as he is finding complex, challenging roles to get his hands dirty. Who says you can’t tarnish that One Ring to rule them all? The digging Mortensen’s been doing in the last two films has been under the auspices of acclaimed director David Cronenberg, the provocative filmmaker who’s examined the dark side of Mortensen’s characters in “A History of Violence” and “Eastern Promises,” which opens in wide release Friday.