The Alice in Wonderland actress will appear in Saturday's episode as Queen Annis, a foe of King Arthur (Bradley James).
"She's been the Queen but she's been with her [late] husband who's done all the fighting and being on the front line." Duncan explained. "So they mirror each other. Death has brought [both] Annis and Arthur to the position they're in."
She added: "Both Annis and Arthur find themselves in a situation of having a great deal of power and having to take responsibility of that power without the experience."
Duncan explained that Annis and Arthur will clash after the death of her husband Caerleon (Steven Hartley).
"Her decision-making that could very easily be floored by the fact she's fuelled by anger about the injustice of Caerleon's death," said the star. "She's very, very angry."
The fourth series of Merlin has continued to be a success for the BBC, peaking at over 7m viewers and scoring the highest AI (Appreciation Index) figures since the show began in 2008, rating as 88, 90, 89 and 89 again over the past four weeks.
"We are thrilled that Merlin is achieving such fantastic audience appreciation scores," said executive producers Johnny Capps and Julian Murphy. "The opinions of our viewers are highly valued."
The fantasy drama continues this Saturday at 8.10pm on BBC One.
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Watch Merlin star Katie McGrath talk to Digital Spy below: