He told TV Guide that the AMC series will continue to deviate from the original comic, in which the character died after just six issues.
"I was really surprised how quickly Shane bought it [in the comic]," he explained. "Once you start fleshing these characters out... you really are loath to get rid of them too quickly."
Bernthal himself joked that he was "relieved" by Darabont's decision to extend Shane's story arc.
"I'm glad I still have a job," he said. "There's a lot more that they can squeeze out of this character."
Comic creator Robert Kirkman previously claimed that Shane's prolonged televison life makes him a "complete unknown for [comic] fans".
The Walking Dead will return to AMC in late 2011.