It seems almost too on-the-nose to cast Christopher Heyerdahl as Marcus, one of the three leaders of the Volturi -- the all-powerful masters, and enforcers, of the vampire world. But it's interesting to watch Marcus in the Volturi scenes in New Moon (and when he crops up again in Breaking Dawn).
Marcus is at once the most perceptive of the vampires (he has an ability to read the emotional connections between people) and one of the most damaged: the loss of an old love made him turn away from the emotional expression he so easily reads in others. His character's main trait, apathy, is transformed by Heyerdahl into a kind of anguish -- an accumulated dread of action that seems to reflect a saturation with grief that has only intensified over the centuries.
What's particularly interesting is that Marcus is written as a teenager in the original novel -- he was 18 when he was made a vampire -- and his youthful appearance was intended as a simple contrast to his jaded demeanor. But director Chris Weitz swapped the character's physical appearance with Caius (who was written to be older), chosing to draw off of Heyerdahl's capacity for hinting at unseen reservoirs of emotion.