The Irish actor told Entertainment Weekly that the new episodes will focus more on the show's core characters.
"It was always going to be a growing-up tale for all of us," he said. "We do have some brilliant guest stars - we've got Liam Cunningham coming in - but the stories are more focused now on the legends that we're going to become."
Morgan's co-star Katie McGrath (Morgana) agreed that the new series, expected to air on BBC One in late 2012, has a more adult feel.
"It's a three-year time-jump, so... we're all older," she explained. "We were playing... very early 20s at the start, now I feel like we've all started to play adults, which for us is a great change."
Game of Thrones star John Bradley will also appear in series five of Merlin, while Anthony Head will reprise his role as Uther for a guest appearance.
> Merlin exec: 'Series six talks have begun'
> Merlin exec talks 'sophisticated' series five, Mordred recast
Watch a trailer for the next series of Merlin below: