Scifiwire.com has a great interview with Christopher Heyerdahl, who will be playing Marcus in the Twilight Saga. From the excerpt below you will see that he confirms his contract includes Breaking Dawn -
"Heyerdahl adds: "Here's how I approach him. When Marcus is brought in, he's there against his will. He and his love tried to leave an awfully long time ago. Through a series of tragic circumstances, the love of his life was killed, and he's now being forced to stay and use his powers for the betterment of the Volturi. He's not exactly there, waking up and looking forward to eternity. When Edward and Bella come walking in, his ability to sense the power of their connection is something that wakes him up, brings him back from the boredom of that eternal life and sparks something in him that brings Marcus as close to excitement as he can get. That idea has to be contained and expressed as little as possible to his rather power-hungry captain-in-arms, Aro, which makes for a rather interesting conflict. You might not know it, but Marcus is pretty darn excited by this love that he witnesses. At the same time, it causes him great pain with the memory of what he had and lost. It was very complex to play, especially when you are doing it with so few lines. It will be interesting to see how it is cut together."
Heyerdahl—who has worn extensive makeup and prosthetics for some of his roles —said that the makeup required in New Moon to transform him into one of the undead was easy by comparison.
"Yeah, they do their best to suck the life out of you, but certainly nothing compared to any of those past prosthetics," Heyerdahl said. "From that perspective, it was pretty easy. The looks they are getting in this film are absolutely fantastic. Everyone looks gorgeous and deliciously lifeless."
Marcus also plays an important role in the fourth Twilight novel, although Heyerdahl hasn't peeked that far ahead yet. "I have left that book alone," he confirms. "I want to leave it until we get closer to the movie. I was talking to Stephenie about it, and she asked if I had read the final book. I said 'Nope, I don't want to read it yet. I want to leave it alone until we get there.' 'Oh, just you wait. You'll love it.' I do know generally what happens, since it is impossible not to find out. And, yes, I've been contracted for the final chapter."
You can read the full interview here.