Dean was unconsciously thinking the same thing as Sam. Grab Chuck by the scruff of his bathroom, drag him into his house, demand answers, burn some books, and get out. He hadn't been expecting this man to be Chuck. Dean wasn't exactly sure what he had been expecting but he was damn sure that Chuck wasn't it.
"Problem?" Dean repeated, brow quirking slightly. Yeah, there's a fucking problem! He wanted to shout. My whole life is in a fucking book! That's your damn problem.. Instead, he pasted a smile on and said. "Sorta. We came here because we wanted to talk about your books. About the Winchesters." Because who knew. Maybe Chuck wrote books about a group of sisters that fought demons and -- Wait. No. Dean shook the thought out of his head. He was tired. Focusing he continued, trying for a little less stern than Sam. Even though he still was contemplating the pulling by the scruff of the neck.
"Could we come in?" Pause. "Please?" See. He could be polite and have manners.