The Heroes actor will play Adam, a member of a magical coven, in the new CW drama.
"I don't think Adam would... choose to be a witch," Dekker told Collider. "But he's also probably the most informed about how their powers work."
He continued: "In many ways, [he] becomes Cassie (Britt Robertson)'s instructor. But, of course, that also brings them closer together, which is dangerous."
Dekker also suggested that Adam's loyalties could be tested, hinting that no character on the show is wholly good or bad.
"The beauty of this show is that none of us have to stay stagnant," he explained. "We all have the possibility of any one of us turning good or turning evil, at any point. That's the freedom of the storyline."
Dekker, who also starred in Terminator: The Sarah Connor Chronicles, added that he has "an affinity" for the science fiction and fantasy genre.
"I love genre works," he admitted. "I like watching them. I like reading them. I've probably only done two [sci-fi or fantasy] projects that I've been offered, or that I looked for. It's just work that comes my way."
The Secret Circle will premiere on September 15 on The CW.
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Watch a preview of The Secret Circle below: