The actor previously suggested that there is a "duality" to his interpretation of the character.
He told reporters at the recent TCA press tour: "Merlin is sort of Machiavellian in his architectural bring-about of this young king (Arthur). He walks with a very dark shadow as well."
Fiennes described Merlin's relationship with the scheming Morgan (Eva Green) as "villainous antagonism".
"There's great chemistry," he said. "It's essentially good and bad, but you realise where her evil roots stem from."
He also confirmed that the use of magic will hold "an immense cost" in the forthcoming Starz series.
"If you dabble in [magic], you will be spiritually [and] physically drained," he explained. "If you mess with that kind of power, it can destroy and ultimately take your life."
Starz will air a sneak preview of Camelot on February 25 and the series will officially premiere in April. The show will air on Channel 4 in the UK.