Clarisse comes from a family that celebrates free-thinking; they don't sit and watch television like every other family, but sit and talk about issues. She explains that people don't actually talk about anything except what they see on television; they speak about brand names, and shows, and just repeat things that everyone else repeats. When she speaks about this to Montag, he is initially confused, but realizes later that it is entirely true.
"And the words I did hear I didn't understand!"
"...Even their names helped. Compare Winston Noble to Hubert Hoag for ten seconds and you can almost figure the results."
"Damn it!" cried Montag. "What do you know about Hoag and Noble?"
"Why, they were right in that parlour wall, not six months ago."
(Bradbury, Fahrenheit 451, Google Books)
At one point, Montag stands outside Clarisse's house, and hears her family talking about substantial issues, not just the vague impressions they would get from television. This sort of conversation is almost unknown in this society, and so when Montag's mind becomes expanded through reading, he finds that he has nothing in common with most other people.