ReelzChannel has a couple of awesome rundowns of who's who in New Moon, like the one above. Check out both the New Wolf Pack Guide and the Volturi Guide.
Where you've seen him: In Smallville, Stargate: Atlantis, and Supernatural. This guy likes his TV!
In New Moon: Marcus can read relationships, which is handy when you're a leader of an ancient vampire clan. Nothing gets past him.
Verdict: Mean-looking and intimidating, he'll be great as bored, menacing Marcus.