"It happens by a sure twist of fate," Singer told E! Online. "The Cas that they find is not exactly the Cas that left them."
He also promised that viewers can expect more twists and surprises as the seventh season of Supernatural continues.
"We usually will end [the season] with some sort of a cliffhanger, which will inform the next year," he explained. "Then [the writers] may get into a room and say, 'OK, we've painted ourselves into this corner, now how do we paint ourselves out?'"
Singer added: "There are some surprises in the last few episodes that I think people will be happy with."