Johnny Capps told SFX that the BBC fantasy drama will deal with more mature themes when it returns later this year.
Series co-creator Julian Murphy added: "It's a bit more adult and it takes the characters to new places. They're older.
"Take Gwen - she has to be a mature queen of Camelot now and you see stories where she plays the politics of that role in a very sophisticated way."
Angel Coulby (Gwen) recently told Digital Spy that her character is "a lot more assertive" in the upcoming episodes.
"I think, having in the past been a little bit unsure of herself and never quite believing her worth, now she has accepted that she's good for the job," said the actress. "She's fulfilling the role very well - she's very fair and just."
> Merlin star Colin Morgan: 'Arthur will be forced to confront magic'
> Merlin: Colin Morgan wants 'Lord of the Rings style' movie trilogy
Watch the cast of Merlin discuss series five with Digital Spy below: