The AMC crime drama follows the investigation into local teenager Rosie Larsen's death, in which politician Darren Richmond (Campbell) is suggested to be involved with her murder.
"I think I was surprised by the fan outcry," he told Collider. I watched the Danish version, before I did this series, and they do the same thing. You don't find out until the end of the second season, and there was no outcry over there.
"So, I'm thinking that the big difference must've been that the log line and slogan 'Who killed Rosie Larsen?' must've built a certain expectation. Otherwise, the story's being unfolded exactly as it was conceived to be unfolded. There's not a real difference in the length of the season or in much of anything, except our story is slightly different because we can't have the same killer."
The Killing, which will return to AMC tonight (April 1), is based on the Danish series Forbrydelsen, which is said to be filming a third season.
"I don't know if he might be an outright, absolute, horrible, heinous killer," Campbell said about his character. "He's either this great guy who is a force for good in the world, or he sees himself that way. I don't know. I have to play it as if I could be either thing."
Watch a trailer for The Killing season two below: