However, the Irish actor told SFX that fans will not miss out on the "banter" between his title character and King Arthur (Bradley James).
"Things are serious," he hinted. "There's a three-year gap [from the last series], so immediately you see Camelot in a different state, one that [Arthur's] wanting it to be, and Merlin finds his place in that as well.
"There's definitely a maturing in general of the whole series, but the banter's still there between the two characters because that's who they'll always be, even though their situations are a lot more serious."
Merlin co-creator Johnny Capps previously hinted that the BBC fantasy drama will deal with more mature themes this year.
"It's a bit more adult and it takes the characters to new places," he said. "[The show] needed to grow... up a little bit."
Merlin will return to BBC One next month.
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Watch the stars of Merlin discuss the series with Digital Spy below: