The original plan for Smallville season 9 was for Doomsday actor Sam Witwer to remain on the show and play Major Zod. During season 8, Sam Witwer played paramedic Davis Bloome, a character who suffered from mysterious blackouts. It was discovered over the course of the season that Davis was actually a survivor of Krypton’s destruction, and that he was the human form of a genetically engineered monster called Doomsday, the creature notorious for killing Superman in DC Comics.
In the Smallville season 8 finale, black Kryptonite was used to separate Davis from Doomsday, but in the end, both characters were killed. Once the Doomsday story was over, Smallville looked to Zod for its season 9 villain. The real Zod had already been defeated in the season 6 premiere, but the show found a way to introduce a new version. Released from an interdimensional prison in the season 8 finale was an army of clones; leading the group was a young Major Zod, played by Callum Blue. This clone was created from the original Zod’s genetic material prior to him assuming the role of general.
Before Callum Blue was cast as the character, the producers intended for Sam Witwer to play Zod. Doomsday and Zod are, of course, two separate characters, but the writers had a way for it work. Clearly, this is where Smallville wanted to go, as Witwer did portray Zod (without showing his face) in a brief shot in the season 8 finale’s cliffhanger ending. In fact, Witwer playing Zod was something they set up in a season 8 episode when they explored Doomsday’s backstory. It was said that Davis was the genetic “son” of Zod; if he was created from some of Zod’s DNA, it was possible for them to look similar.
Smallville didn’t proceed with this plan because Witwer turned down the opportunity to remain on the show as Major Zod. He said their explanation about him looking like Zod didn’t make sense to him, and he felt that everyone in Metropolis knowing what Davis looked like and that he was a serial killer would create a narrative issue. As a result, Smallville abandoned the Doomsday-Zod connection, and looked for a new actor to play the villain. That’s when Blue entered the picture, and became the show’s new Zod.
It’s likely for the best that Witwer didn’t continue on as Major Zod. With Davis dead, there was really no need for the actor to remain associated with the series. Zod looking exactly like Jimmy Olsen’s killer and the paramedic who turned into Doomsday wouldn’t have served an obvious narrative purpose, and for that reason it just would have created unnecessary confusion. Putting a different actor in the role allowed Smallville to leave Davis behind completely, and move forward with an all-new villain to challenge Clark.