The actor will appear as the wizard Merlin in the Starz fantasy drama, which premieres tonight at 10pm.
He told Assignment X: "When I was first offered this, I thought, 'Merlin?' And then, 'Me [as] Merlin? Merlin's [got a] big beard, staff and pointed hat, and why?' It wasn't how I felt I would be attracted to the part."
Fiennes added that he ultimately accepted the part after becoming intrigued by the character's "wonderful, mercurial, Machiavellian range".
"I felt this man was not to be trusted [and] is full of duality," he suggested. "I also wanted as an actor to have fun and discover the range. I wanted to tap into that."
The former FlashForward star explained that he is now interested in the possibility of filming a second season of Camelot.
"There's so much to be tapped into [in terms] of source material," he said. "It's never-ending. But we have to wait and see. There has to be an ongoing kind of romance and dialogue about how we evolve it."
Showrunner Chris Chibnall previously confirmed that he has planned for several seasons of Camelot.
> Eva Green: 'Camelot role is complex'