In a video released by Netflix on Tuesday, the Stranger Things star admits that once his character began to treat Eleven as a daughter, he just couldn't be up-front about her abilities.
"I don't think he can actually view her powers in the same way that other people view her powers," Harbour said. "Because Hopper is looking at her to be a little girl that he can care for. So when she throws the book with her mind, or when she breaks windows, he can't take that in, because it would be too terrifying."
Eleven was held prisoner in a government lab that experimented on her to try and control her powers, so finding a caring father figure with Hopper is new to her. But it's also rejuvenating for the police chief, who lost a daughter of his own under circumstances that still aren't quite clear.
"So you have this guy who is alive again, and he's found purpose, and he's found, sort of, (the) will to live," Harbour says. "And he has the opportunity to create more justice in the world for this little girl."
Stranger Things is expected to return for a third season in 2019.