Ritchson will reprise his role as Aquaman in the show but told People that his role is much "darker" now.
"It's exciting to me," he said. "I was waiting for [the producers] to tell me to pull back a little and stay lighter like the last four times around. But it was nothing like that. It's getting gritty."
He added: "Aquaman's got the weight of these new responsibilities that he has as the new king, with his new wife, the queen. It's this different side of him than I think has been seen in the past, so it was fun to play that."
Ritchson also suggested that fans will be happy with the show's final season, saying: "Loose ends tie up and all this good stuff comes to fruition."
Smallville continues on Fridays at 8/7c on The CW.