The sixth season of the show will begin with Dean having settled down with girlfriend Lisa (Cindy Sampson) and her son.
The actor told Zap2It that he was frustrated with the decision to turn his character "soft".
"There was a big shift in Dean's character... much to my chagrin," he explained. "I was not happy with it. Dean was really kind of being written soft. We spent five years with this guy being a tough, shoot-first-ask-questions-later kind of guy and now all of a sudden he's lived one year in more of a domesticated life and he's gone soft on us."
Ackles revealed that he had attempted to alter Dean's characterisation through his performance.
"It read pretty soft on the page, but I think I toughened it," he said. "I had no idea what the hell I was doing. Even the dialogue, the way it was written, [was] so soft and affected."
The sixth season of Supernatural begins this Friday at 9/8c on The CW.